• There’s a couple I use a lot at work that haven’t been mentioned yet:

    Simple Tab Groups is basically multiple workspaces for Firefox. So I can have all my tabs for one project all grouped together.

    Firefox Multi-Account Containers lets you create sandboxed sessions. If you ever need to be logged in to the same site for both Prod and Dev at the same time this makes it easy. Also if you’re trying to diagnose session issues it’s nice to be able to keep things separate. This is add-on that kept me using Firefox even back when it performed worse than Chrome and Google wasn’t as openly evil.