I used Dark Reader until last week, when I discovered a native Firefox setting that does the job better: Settings > Language and appearance > Colors > Manage > set background to Black and override to Always.
No more white flashes, EVER (yes, I tried absolutely everything but on some sites there was nothing to be done, even with every possible CSS hack). And no more add-on speed penalty (to be fair it was small, and Dark Reader is still an amazing tool).
Now the web looks pretty ugly but it is fast and always dark. White flashes banished FOREVER.
I absolutely love Tree Style Tabs. I usually have a ton of tabs open (middle mouse click is my best friend) and that helps me keep it all organized, and quickly close all the ones I don't need anymore. I also did a change in the profile settings for Firefox to get rid of the normal tabs so now I only have the tree ones. (I don't really remember how I did that though, it was ages ago and involved editing some files in appdata)
Tree style tabs, which gives vertical tabs that you can arrange in a hierarchy to keep related ones together
Simple tab groups, which lets you have multiple sets of open tabs you can switch between (can you tell I have a problem with too many tabs?)
Unstick!, which when clicked removes any sticky elements, i.e. parts of the page that stay on your screen while you scroll. It's great for removing all the bars and obstructions to reading that pages like to put in your way. For some reason I have to click it twice for it to work
Read aloud, a good text to speech extension to read pages or parts of pages to you. It can be used with cloud based neural voices from Google and Amazon with some setup
Consent-o-matic, which gets rid of the cookie consent popups for you and it's configurable as to which types of cookies it will refuse or consent to for you
SponsorBlock for YouTube, which can auto skip sponsor reads and various other kinds of segments you select to be skipped
A few short months ago I would have said RES but, well 🤷♀️
Privacy Badger - blocks trackers, rewrites some tracking URLs, etc.
Multi-Account Containers - for those places where you want to keep tabs separate, giving each container its own cookies/session/etc.
Consent-O-Matic - automatically handles a lot of pages that shove annoying (and often technically GDPR-illegal due to lacking a quick "reject all" button) consent forms in your face.
Imagus - shows linked images on hover, including support for galleries and scrolling through all the images contained.
Channel Blocker for YouTube. Stop all those horseshit channels from reappearing in your suggestions.
Control Panel for Twitter. Allows you to customise your homepage by removing / changing parts of the UI, blocking ads and whatnot.
Save webP as png / jpg. Right-click to save those fucking awful files as something you can actual use.
Unwanted Twitch. Add channels / games / tags and keywords to a universal blacklist that stops them from appearing in the 'Browse' or recommended sections. Great for filtering out mince like IRL streams, shit like LoL etc. and chud streamers.
Tab Stash: It lets you organize your tabs into groups and keep the groups around in a sidebar that unloads them when you don't need them at the moment. Very helpful for someone like me who always has a bunch of tabs open.
uBlacklist: It lets you blacklist domains from showing up in search results. It supports different search engines. Every helpful to get rid of SEO spam sites and mirror sites.
Duplicate Tabs Closer: It detects when you have multiple tabs open for the same webpage.
the ones that I actively use: Rotate and Zoom Image; Image extract; SVG Export; Simple mass downloader; PassLok Image Steganography; Color Changer; Save Screenshot; Behind the Overlay Revival
those that work in the background: Redirect AMP to HTML; Chameleon; JPEG XL Viewer; + the usual blockers & security
I often want to reopen a tab i recently closed, so this is very handy for me.
Languagetool
On the fly rule-based Spelling check. Works very good and in many languages. And the best: It's Open Source.
Facebook-, Google- and Microsoftcontainer
Uses the Firefox Tab-Container Fwature, to lock those companies in Tab-Groups just with themselves. I don't use Tab-Groups aside of that, so it comes in handy.
Firefox Translate
(Not really an extention, but still nice) Firefox Gnome Theme, for my personal machines and Firefox UI Fix for the machines at work to make Firefox look more at home on Linux and Windows.
I see a lot of the same addons here, as one would reasonably expect, but I'm surprised there's been no mention of uMatrix. Using uMatrix and denying most elements by default, you can manually allow scripts, media, etc. per domain and save those rules for pages you go to often. It gives you more granular control than simply choosing to allow/deny all third-party scripts, and you can see exactly what's going on under the hood.
LanguageTool (semi open-source alternative to Grammarly)
Invidious (this redirects YouTube links to invidious instance selector)(There are like 4 different ones, all not updated in 2–4 years, I am thinking of learning enough coding to support this)
Tabliss (this is a nice new tab where I can add reminders of what I am
supposed to be doing instead of what I am doing)
uBlockOrigin is a given. Privacy Badger and Privacy Possum I haven't seen mentioned in this comment secgion.
I also use shinigami eyes and dark reader for a more pleasant browsing experience.