Ditto! Unfortunately, it appears that there will be no replacement any time soon, if ever. I am not 100% certain on the details, but apparently, the new WebExtension APIs make certain browser window manipulation impossible. So looks like we're stuck with ESR in short-to-mid term future.
Sadly, I mean a replacement for Firefox altogether. The only plugins I use are Vimperator and uMatrix. I'm posting this from Qutebrowser and I'm very impressed. I just hope I can get uMatrix functionality.
vimfx is the closest replacement at this moment in time.