Firefox has discontinued support for addons that use their native UI components and will only allow newer addons developed as webextensions. So unless the addon developer is prepared to rewrite the addon as a webextension (and who can blame them if they donít, as many are done as a hobby, without any financial reward ) they will no longer run.
Though you probably know all this.
You could try the palemoon browser, as many firefox addons work on this.
If you have to swallow a frog, try not to think about it. If you have to swallow two frogs, don't swallow the smaller one first.