Sorry Mozilla - for the reasons expressed above, 56 will be the last version of your browser I'm able to effectively use given the demands of my browser workflow. If my use-case no longer fits your business case, then we will part ways.
Currently, it looks like my best options are either Waterfox (with its pledge of continued XUL extension support), or Vivaldi.
I can't imagine Waterfox or Pale Moon being able to keep XUL extensions alive for much longer. It was a major maintenance burden for Mozilla, so it's pretty much an impossible task for the comparatively tiny developer teams behind those.
Having said that, Firefox 52 ESR is still supported until June 26, 2018. So, you could use that. And the Waterfox + Pale Moon devs could base themselves on that until its EOL.
Firefox ESR is pointless - it's just delaying the inevitable. I tried Midori, but it runs poorly on KDE. Palemoon is pretty bad - I tried it and it doesn't properly support existing XUL extensions that work fine in Firefox, so it's a no-go in my book. Waterfox, on the other hand, is fast, works exactly like Firefox, and is at least willing to give continued XUL support a shot.
So, it's Vivaldi or Waterfox as best I can figure at this point.