You can stick to Firefox 52 ESR until June 26, 2018, if you want. Mozilla is still working on a more powerful keyboard extension API for WebExtensions and I imagine some extension will be leveraging this at that point.
No, it won't be as powerful as Vimperator, but should be a good bit more powerful than for example Vimium.
When I last tried, the adblocker wasn't as good as ublock. If I were to switch now, I would miss easy access to my passwords, referrer control, zotero, and RES (a bit, a userscript would do).
Some of the adblock issue is possibly an engine problem (there's some pre-script hooks that Webkit-forks don't support).
Password autofill is something I want to take care once there's a plugin API, but there are ways to make it work (the password_fill userscript, autotyping in KeePass*).
Referrer control unfortunately isn't possible with QtWebEngine's API for now...
Zotero won't be possible, but I guess that won't exist any longer with WebExtensions either?
The zotero main program is now a standalone thing, but there's a webextension that lets you add things to your library. Looking into it, it seems you can get a rudimentary experience with a bookmarklet/jseval:
I'm glad that Greasemonkey support is coming, though. Will the GM scripts run in the same JS scope as page scripts?
Webkit forks also render text slightly worse for HiDPI screens and don't seem to support CSS `hyphens: auto`. Which is annoying because I really like justified text.