Mozilla's decisions will lower customizability/popularity

Firefox 57 will support only addons written via the WebExtensions API, which will mean that a great deal of well-loved Firefox functionality will disappear. This includes, e.g. Omnisidebar and Keefox. The community are furious (see here, here and here).

Still the most private for general browsing

Despite this, Firefox the best browser for customizability and privacy. It is the only browser able to give the user a huge range of options on everything from WebGL to WebRTC, changing how many closed tabs are remembered/can be re-opened, whether websites can ask for information about copy-pasting, battery status... If you don't know about this, you'll be surprised.

I recommend following the privacy guidelines set out for configuring Firefox on privacytools.io.

Firefox is open source and remains the best option for general online privacy, short of using Tor Browser (which is based on Firefox, btw.)

The main reason for many, if not most, people to use Firefox over other browsers such as Chrome or the new Opera, is customisation. No other browser comes close in the diversity and power of its extensions ecosystem, and it's really why I personally still use it as my main browser. While a number of developers do their best, Chrome's selection of extensions is, to put it bluntly, laughable. The average quality is terrible, many developers won't even try because they know already how ridiculously limited the possibilities are that Google gives developers.

Sadly, over the past few years, starting at around the time of Firefox's rapid release cycle, most extensions I installed were to restore some functionality that was removed. But thanks to the powerful possibilities given to extensions a good workaround could usually be found. For the benefit of anyone who might be interested, here is my short list of Firefox extensions that I find absolutely essential.

Category 1 consists of 4 extensions which, in my humble opinion, are essential to turn Firefox into a usable browser in the first place. To be frank, I think any browser should just come with this functionality by default. Note that this is not a criticism against Firefox per se – no other major browser offers these things, at least Firefox is one of the few (the only one?) that allow you to add them afterwards.

• NoScript: Browsing the web without this is just a liability. You're protected from most web-based vulnerabilities and exploits by default with NoScript, and the most annoying ads to boot. You will never go back to the usual "doors and windows unlocked and wide open" way of surfing the web. A slight inconvenience in the beginning by having to whitelist trusted servers, but it will very quickly pay off in a surfing experience that is about 1000× faster and 1000× safer and 1000000× less annoying. And websites that, without good reason, don't even work fully unless JavaScript is active? Ditch them, you don't need that poorly engineered mess in your life.
• Nuke Anything Enhanced: Remove annoying, broken, and unnecessary elements from web pages. Very useful for preparing pages for print, but not only.
• Textarea Cache: What the old, abandoned and now non-functional Lazarus extension used to do: store backup copies of whatever you write into webforms, so should anything undesirable happen – browser crash, accidentally navigating away from the page, or a failure to submit on a website that is so poorly written as to lose your data when it happens – you can restore it from the cache. Can save you massive amounts of time and frustration.
• Classic Theme Restorer: After the Australis redesign, UI customisability was severely restricted, if not eliminated. Most of its elements are also a massive waste of space, making Firefox almost impossible to use for power users. Do you sometimes open more than 10 tabs? Do you have more than 100 bookmarks? If so, then you can't really use Firefox unless you let CTR fix some of the damage done by Australis. Thanks to it, I have a title bar that actually shows me titles of pages I'm on, I have more space-efficient (and square!) tabs, prettier buttons and toolbars, and almost most importantly: so much less wasted screen real estate all around.

Category 2 is a more personal selection. These extensions I wouldn't consider essential, but using Firefox without them would just be endlessly frustrating to me. These are real time savers and effective grey hair preventers.

• Bookmark Favicon Changer: I use a bookmarks toolbar without names or text labels (easy one-click access to many essential sites!), so favicons are all I have to distinguish them. Sadly, some websites don't offer one, or a very much less than expressive one. This extension allows you to set custom ones. Because of differences of opinion with Mozilla, the extension has to be downloaded from the author's own website.
  
• DownThemAll! I thought the days of needing download managers are over, but this is a nifty little helper at times. Poor servers can make it very helpful to be able to resume downloads, and its bulk download options are very handy.
• Keybinder: Because Firefox doesn't allow customising keyboard shortcuts. I don't know what was the brilliant idea behind disabling the Escape key in Firefox (it used to cancel pending requests, including AJAX), but it was terribly idiotic, because now you're no longer able to just bash escape if something's happening that you don't want to – the most common use case being, of course, hitting a link by accident. Set up the Escape key to mean "Stop" again using Keybinder (as otherwise the key is completely non-functional) and you can prevent that stuff from happening again. I also use it to disable the F12 shortcut for the developer console, because I keep hitting it by accident.
• Keyword Search: Firefox always supported multiple search engines. It was completely incomprehensible why they would make the search bar completely superfluous by forcing users to always search with the one engine they currently set as a default, regardless of where they're searching from. In other words, searching from the search bar is now functionally equivalent to doing it from the URL bar, and I honestly don't know why the former is even still there. Since I don't want to have to constantly switch the default engine back and forth, I set Keyword Search up so that the URL bar searches my default engine, and the search bar searches whatever engine I select in it. Sorted, searching is usable again.
• Link Visitor: This is more of a personal choice; I often use unvisited/visited link colours to remember what I've already read. If I click something by accident, I can toggle the link colour back to Unvisited using this extensions.
• Tampermonkey: Customise the web.
• URL Flipper: Very handy to navigate paginated websites, galleries, and so on. Allows you to increment and decrement URL variables easily.
• Video DownloadHelper: Say no to media conglomerates trying to take away your consumer rights. You have a right to download private copies – exercise it! And disable those abominable EME DRM extensions in Firefox settings, while you're at it. Say no to media that is "defective by design".

After a couple of years of Mozilla "improving" the browser by bloating it with various silly features, they finally made a huge step forwards and went to make Firefox fast and efficient.

Firefox got a huge modernisation of both backend and UI in version 57 (Quantum), which will be released in November. Firefox now takes advantage of multiple processor cores, prioritises tabs, and perceived performance is also improved. When you try Firefox Beta, you actually feel the new speed.

I really recommend everyone who had become fed up with Firefox and switched to Chrome to give the new version another chance.

Mozilla is a great organization, and Firefox is more stable and less memory-consuming than Chromium-based browsers like Chrome or Opera. I also generally like Firefox's library of add-ons more than those in the Chrome Web Store. NoScript is very useful.

I don't really like what Mozilla is doing with Firefox these last few years (Pocket, telemetry, data collection, the Looking Glass addon, the removal of individual cookie management recently and many other craps…) so I'd recommend to everyone to start using Waterfox instead.

It's fast and very pretty, very customizable

I use Firefox everyday , and I really like it

Finally, with Firefox 43, an official 64-bit version of the program is available for Windows :D

Lots of users experience Adobe Flash Player hangs or crashes. I don't know why it isn't fixed yet, but there is a simple workaround that makes Flash work properly: http://mzl.la/1zuyG1P

Try it - it solved the problems for me!

Try http://alternativeto.net/software/classic-theme-restorer-customize-australis- . It restores classic Firefox button, tabs shape, removed customization features, etc.

[Edited by Venom88, March 17]

It has good rendering time and fast starting time but version 32.0.2 got the best score so far.

If Firefox is using too much memory, just restart it and your tabs will come back. The addon Restartless Restart is great, as it adds a button and a keyboard shortcut for restarting.

Also bear in mind that the more RAM you have, the more Firefox is going to use.

[Edited by Venom88, April 07]

Firefox is currently the best alternative browser at the moment. My only gripes is that it has its archaic download window instead of the streamlined Chrome like downloader. While Google Chrome is better out of the box, it suffers greatly from weaker Addon support. Firefox's addons are much, much more useful.

Chrome addons implementation is rather poorly done. It slows the browser down and increases the memory usages/load times dramatically compared to Firefox. Yes addons slow both down in some way or another, BUT Firefox's addons are closer to the browser so that they can have more functionality.

Last but not least, Chrome is the ad brokers browser. Everyone using Chrome has to understand why Google sees this as a huge opportunity, right? But I must say Chrome a great browser in it's own right. Much MUCH faster with most Javascript.

I have a lot of problems with Firefox plugins, especially with videos in the browser.

If I want to see a streaming video I MUST install VLC (80 MB) with its plugin (which doesn't always work) and if I want to see a video file, like mp4, mpg or avi in Firefox, I MUST install Quicktime (73 MB) with its plugin.
But since I use Media Player Classic Home Cinema, I don't use VLC and Quicktime: is there a way to install ONLY their plugins to view videos in Firefox or another way to do it?

There is also Windows Media Player plugin installed in Firefox: can a plugin of these handle everything about videos, streaming and video file?