A bunch of other key extensions already listed, but what I don't see that I use often is redirector. It lets you put in regex patterns to redirect window locations to another destination. For example I've used it to redirect any reddit.com link to old.reddit.com, use it to remove tracking parameters from URLS, redirect youtube to yewtube, etc.

For sake of the thread though, here's a short list I have on all systems

• Redirector
• Ublock Origin
• Firefox multi containers
• Bitwarden
• Ghunt companion
• Leech block
• Noscript

Used to have

• RES
• Modtoolbox
• Privacy Badger
• HTTPS everywhere