Tor Browser is the single best option to use for staying anonymous online. The strength of using tor browser in particular to access the Tor network is that the browser is designed to have a very uniform fingerprint across all those who use it. For this reason, it is not recommended that users change the add-ons that are installed. Tor Browser is built on Firefox with NoScript and HTTPS Everywhere installed by default. The version of tor browser built into Tails OS additionally uses uBlock Origin, so this is the add-on that carries the least risk of increasing your tracking presence if installed by the user.

The Mozilla Foundation is a non-profit organization which has supported the open-source community for over two decades. They maintain the Firefox browser, which remains the best option for everyday use for most people (though you should use Tor Browser when you can). Firefox can be compiled from the source code, meaning anyone can see exactly how it works to ensure that there aren't any nasty hidden trackers and to find security flaws so they can be patched. Additionally, Firefox is the only major browser (other than Safari, I suppose) that is not built on Chromium. Although Chromium is also open-source, it is maintained by Google, perhaps the worst internet privacy offender out there. See https://www.privacytools.io/browsers/#about_config for about:config hardening tweaks.

Basically Chrome with greater customization and transparency, minus Google telemetry. Although Firefox should be preferred, this is an option to consider for those who dislike the Firefox UI.

An open-source and Chromium-based browser with some security upgrades and built in ad-blocking. Although it is possible to use Brave to access the Tor network, Tor Browser should be preferred because Brave will not offer the same level of fingerprinting resistance. This browser performs well on tools like EFF's panopticlick (https://coveryourtracks.eff.org/), but many privacy advocates turned away when Brave was caught autocompleting affiliate links for users trying to access Binance. Users can opt-in to receive banner ads that will generate cryptocurrency upon viewing which can be donated to approved content creators (see: https://brave.com/brave-rewards/). Although I do not see myself using Brave as my daily-driver anytime soon, it may a good option for people who are less invested in understanding browser add-ons and I do think the Brave Rewards program is a clever idea that I hope will get more traction as time goes on. I certainly wish everything could be FOSS, but it is a sad reality that monetization is necessary for projects to have teams dedicated to security patches and improvements -- services like Brave which find ways to generate revenue without selling out user data will be important for mainstream adoption of privacy-preserving technologies.

ESSENTIAL. The best ad-blocker out there, fully open-source, developed and maintained by Raymond Hill who has been advancing the privacy-centered and open-source software community for years. It is worth your time to learn how to use advanced mode. Also be sure to disable WebRTC.

ESSENTIAL. Developed by the Electronic Frontiers Foundation (EFF), this add-on will make sure you are always accessing the HTTPS (encrypted) version of every site you visit. It can also be set to deny any unprotected connections.

ESSENTIAL. Another add-on by EFF, this one will help block trackers and unnecessary cookies.

ESSENTIAL FOR HIGH THREAT MODELS. JavaScript is all around us, but can be a nightmare for privacy and security. It is often inconvenient to block JavaScript because it can break sites, but if you are serious about taking all steps you can to reduce the risk of being tracked online, NoScript is a must.

RECOMMENDED. This add-on can block requests to centralized content-delivery networks, such as Google. You can still access the same sites you normally would, but with a lesser risk of being tracked.

USEFUL. Another cookie management tool, this one allows you to create custom profiles that will control which cookies are used. For example, you might want the cookies for your work email to be isolated from those that let you sign into your personal email. As the name suggests, you can also easily create temporary containers that will be forgotten when they are closed.

USEFUL. Sometimes you need cookies to access certain sites, but there's no reason to have them following you around elsewhere. This add-on can be used to delete cookies automatically when they aren't in use.

USEFUL. This add-on can spoof information about your browser's user agent, such as default language, screen resolution, operating system, timezone, and more. All of these pieces of information make you fingerprint more unique so it can be useful to give out fake info and change settings often.

USEFUL. If you do need to use JavaScript to access a page, this add-on can alter API calls which will prevent fingerprinting in some cases.

USEFUL. If you ever notice the url you are going to is super long and includes things like "UTMsource" or "redirectFrom", it's because the url is storing additional information about how you navigated to it. ClearURLs will automatically redirect you to the shorter version without the unnecessary tracking information.

USEFUL. Some people swear by ghostery, I personally do not use it often because I am not convinced it offers greater functionality that uBlock Origin + Privacy Badger.