Happy to see that Mozilla was serious about their intentions to extend the WebExtensions API in post-57 Firefox. I'm also happily surprised at the section about user notification of extension overrides; having a nice UI for this sort of thing is a difficult yet worthwhile problem (and I'm also a big fan of the new Preferences tab in general, it works far better than the old popup window). Hopefully some of this will…
Yep, me too. I switched to Waterfox for the time being because of how important FireGestures is to me. If full-fledged mouse gestures extensions become available in Fx again, that would be a big step towards making the new Firefox usable for me.
Since you mentioned vimperator, another major issue for me is the lack of a VimFx style extension - that one is much simpler than vimperator or pentadactyl, and has other new attempts at alternatives, but 'Saka keys' is the one that seems to come closest (and makes this less of an issue than the mouse gestures one) and that's still kinda buggy and years behind VimFx in terms of features.
I would have missed Tab groups though it's not that big of a deal to me, but I'm glad we'd have that feature again. And another minor issue in that league is the lack of a good Speed Dial extension with a decent UI and set of features (which none of the "replacements" had last time I tried them out).