Firefox is phasing out all extensions using its old architecture. With the upcoming Firefox 57, only ones written as WebExtensions will be supported. If that's a problem for you, you should switch to the ESR release, as I did. FrontMotion has MSIs for Firefox ESR 52 and old extensions are supported there. The last planned ESR 52 release is in May 2018, so that's how much extra time you have to find replacements. You'll still get security updates up until that point, just no new features.
XMarks does have a beta version written as a WebExtension so that should work eventually. Since I'm on the ESR release I'm just going to wait, but when I do switch I'll see if it's possible to recreate my edits on it. I haven't looked into it yet.
Firefox is also phasing out all web plugins other than Flash. This is a problem because IETab uses that to function. There's no WebExtension version of it that I'm aware of (even though they do have a Chrome version) so that's another reason to just wait and use the ESR releases for now. If that doesn't change before the next ESR release I may just have to abandon it. Oh well.
I added this setting to my configuration script to make non-Flash plugins work a little longer. It lets IETab work in ESR 52:
"plugin.load_flash_only", false