Applied the hotfix to my 54.0.1 (64-bit) version, and it seems to have done the trick!
Maybe not right place to ask, but can anyone tell me for sure whether, now that I got my add-on data recovered, if I finally break down and allow the upgrade path to 56, and then to Quantum and the very latest version, do the upgrades still exist on the Mozilla site to facilitate that? That is, next time it advises me to upgrade to 56, if I say yes, will that succeed? And then the step to Quantum, that will succeed also? Or if, because my current version is “unsupported”, it’s not even supported to upgrade either? I understand many/most add-ons won’t survive the transition, I’m talking just the process itself. And if not, then I guess I’d like to know if I just back up my profile, would the current profile files work seamlessly with the very latest version?