I don't see any way to choose which instance to load. I expected it to at least choose the one with the best ping automatically, but it's either random or just goes back and forth
The Hypothesis toolbar. It adds a comment section to any webpage simply by existing. All those times someone blocked you or COPPA removed YouTube comments is no longer an issue.
More generally, just use Libredirect and avoid using a ton of important but shitty websites. Breezewiki in particular is fantastic, so much faster than Fandom and it'll redirect you to a game's actual official wiki since Fandom SEO's itself to the top even when it has shitty misinformation and isn't maintained.
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.
One that hasn’t been mentioned is I don’t care about cookies. Or something to that name. With it on your never see cookie setting notifications. I’m not super sure if it rejects them all or accepts them all. But it’s nice to not worry about it