For now, there is at least one workaround, but it only applies if you’re using the Firefox Developer or Nightly build. If you look under “about:config” (if you don’t know what this is, you probably shouldn’t try it) and set the “xpinstall.signatures.required” value to False, then your extensions will start working again.
Oh my! Firefox disabled all add-ons because a certificate expired
Well, only those who do nothing do not make mistakes.
Here’s a quick workaround: