If you habitually have a lot of tabs open, you'll probably know how annoying it is finding things when each page title has been condensed down to 4-5 characters. On widescreen displays (especially 16:9), vertical pixels are also a lot more precious, while horizontal ones are plentiful.
For me (3840×2160 display, 200% scale), its vertical tab sidebar fits about 30 tabs before needing a scrollbar, and you get a full width title for each and every one.
It can be a bit of an adjustment at first, but I've been using this since the pre-WebExtensions days (since around Firefox 4.0), it's definitely one of my must-haves.
First of all, install Betterfox, it is not an extension per se, but a set of custom settings. Betterfox offers a lot of fixes, including removed Mozilla telemetry, increased speed, smooth scrolling, privacy protections, etc.
I believe, Betterfox can also be used on Floorp (In case their website doesn't work, here's a Github link - https://github.com/Floorp-Projects), which is a Firefox-based browser with Vivaldi/Opera-like interface. Sounds neat, but i didn't really test it yet.
In case you feel too lazy to install Betterfox/Arkenfox/other user.js modifications, you can use Librewolf instead. It is a version of Firefox with bundled Arkenfox and uBlock Origin.
If configuring Redirector is too confusing for you, you can use LibRedirect, it can automatically redirect YouTube to Invidious and Piped, Fandom to Breezewiki, Google to SearX, Twitter to Nitter, and so on, so you won't have to bother with popups and ads.
For password managers, use Bitwarden, if you want to have your passwords synced in the cloud, or KeepassXC, if you want to store them locally.
Block The Rich is a fun little extension for those who are tired of reading billionaire spam, but i did not test it.
Instance Assistant Is made to improve Lemmy and Kbin experience, but, once again, i did not test it.
Also, do not use Google, Bing, Yandex or Brave as your search engine, instead switch to DuckDuckGo, Mojeek, and 4Get or SearX
Ublock Origin
Enhanced Steam
TWP - Translate Wep Pages (works better than the native chrome version)
ytc filter (Youtube live chat filter. When general chat becomes spammy)
Tabliss (Better version of the chrome start page)
Camelizer (Amazon price history)
Return YT Dislike
Dark Reader (How could I forget that...)
Bitwarden (or password manager of your choice)
As well as those others have already mentioned, I use:
Linguist is a translation extension that respects your privacy. If you switch to the Bergamot translator it acts in offline mode.
Redirector which allows you to set custom rules so you can redirect (for example) Twitter to Nitter, Instagram to PikUki and also rules to redirect pages that are behind paywalls via 12ft.io (or 1ft.io as 12ft is currently down) .
TamperMonkey a userscript manager. I don't have loads of userscripts but I do have things like SocialFixer for making FB a bit better, Absolute Enable Right Click and Bandcamp Volume Bar.
Honestly the less you can live with the better. For me essentials are always ublock for ads, imagus for better image viewing while browsing and simple translate because I deal with a lot of languages. Also containers is useful for work.
Lemmy Instance Assistant It does things like if someone links a post and the link takes you to the post on another instance, it adds a button to show the post on your home instance. You can also right click on a page (say, an article on a news site) or image and choose the option to share it on lemmy, which creates a new post. It also has stuff to help you when you click a link to a community but the community is not federated to your server, or you can go to the list of communities on another instance and it will have links to take you to that community on your home instance. That sort of thing. Basically the beginnings of a RES for lemmy.
I also like Dictionary Anywhere, which lets you double click on a word to get a definition, a bit like the one Google one for Chrome.
There are also various container extensions such as a Facebook or Google one, that isolates those sites to attempt to prevent that activity being associated with your activity on other sites. It can be a little annoying to get used to but I use them. The annoying thing is that when you click say a google site from a search result on duckduckgo, it closes the duckduckgo tab and opens the site in a google container, but then you can't click back to go back to the search results.
The general container tabs extension is good too. It keeps separate cookies per container. So say if you have 3 different microsoft accounts, you can create different containers. Then you can open a new tab in a specific container and it will remember the account you logged into last time in that specific container, but doesn't affect other containers or tabs not in a container.
I think the arkenfox project website has some good info on this. For a good privacy baseline, you basically only need to enable strict mode and download the ublock origin extension, then change some settings for the extension (e.g. add filter lists).
People have already mentioned the more popular ones
Apart from those, Id recommend Behind the Overlay- it's an extension that removes a lot of unclosable popups on pages in a single click. Things like "disable your adblock" messages or websites that poorly gatekeep content behind a subscription.
A bit offtopic here, but how can I auto hide cookie prompts in uBlock? What I do is that I manually hide them with cosmetic filter, then I never have to worry about accepting them or not (kinda like I still don't care about cookies extension)
If, like me, you do a lot of Chromecasting from your browser, you'll want to install fx_cast. You need to do a bit of manual installation to get it running, but it works great when you do.
SimpleTabGroups - organize your tabs
uBlock Origin - adblocker
Prvacy Badger - block trackers
I still don't care about cookies - remove cookie prompts
Stylus - lets you create your own css based on url
Canvss Blocker - prevents finger printing but can break websites.
I still don't care about cookies is a must have, it is so nice to never have to click those prompts again.
A password manager is also nice. I recommend keepassxc and it's addon
This is a switch I made yesterday, but instead of Chrome to Firefox, I switched from Vivaldi (Chrome, but kinda like old Opera) to Floorp (japanese fork of Firefox with focus on privacy and sane defaults).
I got Sponsorblock for YouTube, uBlock Origin for ads and Gesturify for those awesome mouse gestures (my main reason for using an Opera-like).
Sometimes, I use Tampermonkey for misc Scripts on different Pages, so I installed that aswell. But for now, no scripts for Tampermonkey, as I primarily focus on setting it up to be more like Vivaldi.
Other addons are Ghostery for more privacy and Session Buddy to prevent oopsies when playing around with Tabs and windows.
Floorp even has dark mode for bright pages already included in the box.
Bewarned that the mobile Firefox app is really not great.
I am a chronic tab-user (I have more than a hundred open right now. Yes I'm using pretty much all of them.) and 70% of the time when hitting the tab button it doesn't actually scroll to the most recent tab. I have to tap it repeatedly to get to where I was.
The tab list is horribly wasteful when it comes to space and I see no way to change it.
Some sites also shit the bed completely when auto-filling from my password manager. Like full on freeze the browser or crash it entirely.
When an app opens Firefox in an embedded browser to get you to log in, it will pretty much never direct you back to the app after putting your information in. You have to tap the three dots and open it in the actual app for even a chance that it will properly redirect you.
For 2fa sometimes this doesn't even work. I have to scan my 2fa key then quickly open it in the actual app through the menu before it finishes loading, otherwise it doesn't redirect or gets stuck in a login loop. Fun.
The browser is fine overall, don't get me wrong, there's just some inconveniences that aren't getting fixed.
Surprised noone mentioned NoScript yet. It requires a bit more user interaction. But if you are worried about privacy and maybe security, it is important to know who is running scripts on your machine.
Ghostery hasn't been mentioned - it has a feature to auto-decline cookies. The popup shows up, but you just wait half a sec and everything has been auto declined.