As a recent Chrome uBlock Origin refugee, I was looking for something that'd replace chrome's tab groups.
This add-on does a little bit of that, but differently, and in several ways better.
First off, you WILL want to customize your browser toolbar to put the add-on icon to the top left of the app, right before the tabs. It's a much more natural place to use it.
Second, if you're using dark mode, the default icon is hard to see, and you'll have to tweak a config to fix that. It's spelled out in the "about this extension" page.
Third, if you multi-screen, this is MUCH better than chrome tab groups, since it allows you to trivially move tab groups between windows, and therefore screens.
I like that it can sync the tab groups into a bookmark folder.
The default group icons are awkward, but you can replace them with the favicon of any tab within the group, which solves that.
I just installed this a couple hours ago so I'm still wrapping my head around it, but I like it already.
I've got 12 tab groups already. Just don't expect it to be a drop-in replacement for Chrome tab groups. Take the time to wrap your head around the mental model of this add-on, it's worth it.

*Edit, now with more gripes!:
A dozen tabs or so (about 8% of my tabs) get detached from their group on every Firefox restart, which means on every Firefox update for me. I don't know why. It's not the same tabs each time. I tweaked all the settings I could, didn't help. I have to reassign the stray tabs in their proper groups after a restart, which takes entire seconds.
I'm kinda hoping this is going to stop happening on a future update. If I was a little less lazy, I'd probably try to debug it myself.