Dark Reader - Seriously why can't more sites have a "don't blind me" mode in this day and age.

https://addons.mozilla.org/en-US/firefox/addon/darkreader/

Swift Selection Search - It has a little configurable pop up whenever you highlight a word so you can send it to any number of destinations... search web, search youtube, search imdb, send to translate, look in wikipedia... you just build the search function in the extension and it becomes a little icon in the dialog box.

https://addons.mozilla.org/en-US/firefox/addon/swift-selection-search/

uBo - besides the obvious, it's good for eliminating unwanted design elements from a site.

https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/

Simple Tab Groups - it lets you define groups of tabs as sort of work spaces. I have ones for cooking, working, hobbies... you just configure what you want and dump all the tabs you want into it then you can switch to them like presets. Very handy when you open lots of stuff for a project.

https://addons.mozilla.org/en-US/firefox/addon/simple-tab-groups/

Auto Tab Discard - gives much more granular control of what tabs are slept or set to never time out and how quickly they do that. Good for keeping memory footprint lower if you use a lot of tabs.

https://addons.mozilla.org/en-US/firefox/addon/auto-tab-discard/

Augmented Steam - links to isthereanydeal.com so that whenever I'm viewing steam pages on browser for a game, it's showing me all the current best deals from sellers.

https://addons.mozilla.org/en-US/firefox/addon/augmented-steam/

ScrollAnywhere - more robust click to scroll feature. Let's me use a button to scroll and pan sites more like a touchscreen device while using my mouse.

https://addons.mozilla.org/en-US/firefox/addon/scroll_anywhere/

BitWarden - cross platform open source password manager

https://addons.mozilla.org/en-US/firefox/addon/bitwarden-password-manager/