Post!
My examples will be for Firefox, at least to begin with.
Improve YouTube! (Open-Source for YouTube)
Add custom search engine
Duplicate Tabs Closer
Forecast: Google™ Weather
FoxReplace
FoxyTab
Imagus
OneTab
Rain Alarm Extension
Reddit Enhancement Suite
Reverse Image Search
Tab Auto Refresh
Reverse Image Search
Tampermonkey
To Google Translate
Undo Close Tab
Web Archives
Open Multiple URLs
Unpaywalls
Nano Defender for Firefox
Privacy Badger
uBlock Origin
I still need to test this to see if it's actually working correctly:
Ninja Cookie