Other than multi-account containers, already discussed in the top comments here, another killer firefox extension that I think is largely undiscovered is SessionSync.
I searched for a while for a decent way to deal with the "too many tabs" problem and SessionSync is where I've landed, so far anyway.
SessionSync doesn't ask you to set up an account anywhere new - it just saves tabs as bookmarks into a 'SessionSync' folder in your firefox bookmarks menu, which are then synced automatically with the rest of your firefox profile data, to any other computers or phones synced with the same profile. Even if you decided to stop using SessionSync, you'd still have access to everything using Firefox's usual bookmarks interface. It basically gives you some options that I think firefox was missing, to save and restore whole sets of tabs to ordered lists in bookmark folders rather than dealing with urls individually.
I have no affiliation with SessionSync; just wanted to mention it as one thing I really like in the firefox ecosystem. It has rough edges (e.g. no 'undo' if you save over the wrong folder accidentally, and sometimes weirdly changes the order of tabs on save), but it's the best tool I've found so far to do this sort of thing. Please let me know if you're using something as good or better though :)
I searched for a while for a decent way to deal with the "too many tabs" problem and SessionSync is where I've landed, so far anyway.
SessionSync doesn't ask you to set up an account anywhere new - it just saves tabs as bookmarks into a 'SessionSync' folder in your firefox bookmarks menu, which are then synced automatically with the rest of your firefox profile data, to any other computers or phones synced with the same profile. Even if you decided to stop using SessionSync, you'd still have access to everything using Firefox's usual bookmarks interface. It basically gives you some options that I think firefox was missing, to save and restore whole sets of tabs to ordered lists in bookmark folders rather than dealing with urls individually.
I have no affiliation with SessionSync; just wanted to mention it as one thing I really like in the firefox ecosystem. It has rough edges (e.g. no 'undo' if you save over the wrong folder accidentally, and sometimes weirdly changes the order of tabs on save), but it's the best tool I've found so far to do this sort of thing. Please let me know if you're using something as good or better though :)