Firefox user here.

• Bitwarden password manager
• Bypass Paywalls Clean
• Clear URLs remove URL trackers
• Highlight or Hide Search Engine Results to hide some unwanted websites from search results
• Open in VLC™ media player, useful for some weird streams
• Push to Kindle sends any text article to PDF or to your ereader (not only Kindle)
• Recipe Filter filters recipe pages on blogs and just gets the actual ingredients & instructions
• Redirector for a few paywalls where I use a specific proxy
• RSS Reader Extension (by Inoreader) - as I use Inoreader for following RSS feeds
• Sci Hub Injector adds sci-hub links to many science publishing websites for easy access
• Shinigami Eyes highlights trans-friendly and transphobic social media users or websites
• uBlock Origin
• ViolentMonkey for userscripts

Extensions to be helpful to other people:

• Picket Line Notifier tells you if the website you are visiting has workers on strike - useful especially for ecommerce & news publishers
• Snowflake is not noticeable for me, but allows other people to use my network as a Tor node or something idk
• Wayback Machine archives every page I visit on the Internet Archive.

Fediverse extensions:

• FediAct allows me to boost, reply to, follow, etc. on any Mastodon instance without having to open the right link in my own instance. I wish there was something like this for Lemmy and Peertube.
• Fedishare allows for one-click sharing to several Fediverse platforms, including Lemmy and Mastodon
• PeerTubeify tries to check if a YouTube video you’re watching is also on PeerTube

Youtube extensions:

• Auto HD / 4k / 8k pour YouTube™ - I use it for the environment, so default quality is 480px (because usually I watch the videos on a small side window so it doesn’t change the visible quality)
• Clickbait Remover for YouTube - replaces thumbnails with a frame from the video and makes all titles normally named, no all caps
• DF YouTube (Distraction Free) - removes the homepage & sidebar on videos to avoid rabbit holes
• SponsorBlock auto-skips sponsored segments, intros, credit rolls, etc. on YouTube videos

uBO, of course. note: you guys don’t need ClearURLs with this list added.
LibRedirect for automatically opening Youtube, Twitter, TikTok etc. links in their privacy-focused front-ends. I just make sure to disable all the instances by esmailelbob since he’s a little homophobic shithead
Buster for automatic captcha solving
Consent-O-Matic automatically clicks through cookies banner to deny all the cookies that aren’t necessary, which I like better than just hiding the cookie banner
Redirect AMP to HTML because fuck AMP and fuck Google

uBlock Origin - as ad and script blocker

Dark Reader - for dark mode on every site

Sidebery - for tab management

Firefox:

• uBlock Origin (uBO) - The internet is basically unusable without this. {GPLv3}
• Dark Reader - I like using dark themes and I hate when I get blasted with a light theme when I visit a site. This keeps that to a minimum. {MIT}
• Firefox Multi-Account Containers - It’s nice to keep things separated. {Mozilla Public License Version 2.0}
• Consent-O-Matic - Automatically marks my saved cookie preferences on consent pop ups. This is a great tool to help counter to the dark patterns related to GDPR, but it isn’t perfect. {MIT}
• NoScript - I don’t like giving blanket permission to run JavaScript in my browser. This let’s me choose. {GPLv3}
• Wayback Machine by Internet Archive - Archives the sites I visit automatically and provides a one click option to visit an archived version of a URL that returns 404. Proprietary I don’t know of any alternatives
• Tampermonkey - There are a few very useful scripts that I run periodically. Tampermonkey keeps them organized and easy to run. Proprietary I don’t know of any alternatives
• Reddit Enhancement Suite - I got a lot of value from this extension over the years, but I don’t know how much value it has going forward for me {GPLv3}

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I use the following ones (on Firefox), except for uBO the others are just for conveniency:

• Bitwarden
• Gesturefy (for some time from early 2021 to late 2022 I used to use Vivaldi as my primary browser and now if I’m using a mouse, not having gestures in a browser feels odd…)
• LibRedirect
• Plasma Integration
• uBlock Origin (middle mode and with some additional lists)
• User-Agent Switcher and Manager (if I find a site that says it doesn’t work with Firefox).

One that I love is jumpcutter. Speeds up silences and makes watching long lectures way nice.

If I get back to my PC I’ll send a few more extensions I use.

One I haven’t seen mentioned yet is DownThemAll.

DownThemAll lets you download all the links or images on a website and much more: you can refine your downloads by fully customizable filters to get only what you really want.

Comes in really handy sometimes. (For Firefox / Chrome / Edge)

Another is uBlacklist, which allows you to blacklist domains from Google / Bing / DDG search results (like say, pinterest.*), also for Firefox / Chrome / Edge.

For privacy & security:

• DuckDuckGo Privacy Essentials
• uBlock Origin
• Privacy Badger

For usability:

• Wappalyzer - Technology profiler
• Firefox Translations
• Flagfox
• Grammar & Spell Checker—LanguageTool