Assuming Greasemonkey does not get updated to web extension compatibility...
I'm planning to use Tampermonkey instead of Greasemonkey, one way or the other.
One way: Tampermonkey is a web extension, and has been available for Firefox, though I had poor results when I tried it a year or so back. If Tampermonkey works in Firefox, I'll give Firefox 57 a chance. I note that Tampermonkey is not listed in the "ready for" link on the page you linked to.
The other: I can always just switch to Chrome browser to use Tampermonkey and Port Monkey for HSX; they work well. I already use a Chromebook for 95% of my online activity. I'm no longer a big Firefox booster (I had even dontated $ to the Firefox project, many years ago).
So, don't worry too much. You might like Chrome well enough. And, Tampermonkey is much more polished than Firefox, I must admit.