This eye-care extension enables night mode creating dark themes for websites on the fly. Dark Reader inverts bright colors making them high contrast and easy to read at night.
Open-source password manager storing passwords, notes, and attachments offline with AES-256 encryption, browser extensions, cross-platform support, and TOTP.
MAL-Sync is a powerful extension and userscript, which enables automatic episode tracking between MyAnimeList/Anilist/Kitsu and multiple anime streaming website.
Save web content to your vault with offline access. Secure and private, it clips anything from articles to recipes and integrates seamlessly into Obsidian.
A powerful reverse image search tool, with support for various search engines, such as Google, Bing, Yandex, Baidu and TinEye.
Browser extension enabling one-click saving of full web pages with images, styles, and fonts as a single HTML file for offline access, annotation, and batch saving.
This third-party website gives you better insight into the Steam platform and everything in its database.
uBlock Origin (uBO) is a CPU and memory-efficient wide-spectrum content blocker. It blocks ads, trackers, coin miners, popups, annoying anti-blockers, malware sites, etc., by default using EasyList, EasyPrivacy, Peter Lowe's Blocklist, Online Malicious URL Blocklist, and uBO...
Extension: https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/
Filters: Default + FMHYFilterlist More: https://github.com/yokoffing/filterlists
Violentmonkey provides userscript support for Chrome, Opera, Maxthon and Firefox supporting most scripts for 
Greasemonkey and 
Tampermonkey.
Extension: https://addons.mozilla.org/en-US/firefox/addon/violentmonkey/
Userscript: AdGuardExtra
Web Archives is a browser extension that enables you to find archived and cached versions of web pages. Searches can be initiated from the context menu and the browser toolbar.
With this app, you can easily download files from anywhere. Enjoy fast, free downloads with seamless browser extension support.
Accelerate your downloads by up to 8 times! Internet Download Manager (IDM) is a download accelerator to increase download speeds by up to 8 times, resume, organize and schedule downloads.
Comments
What about
Violentmonkey and 
Greasemonkey ? I don't like having proprietary extensions in my web browser, especially if good foss alternative exists.
Great recommendation, thanks ;)