1. Open a new tab (or use a current one, whatever) and type "about:config" (none of the quote marks are required).Now bear in mind that this disables the new "signed" security feature that's suppose to protect against bad and potentially harmful extensions, so there's a small risk if you're inclined to install new extensions. At best, this is just a short term workaround until those disabled extensions are signed by their creator or Mozilla allows them to be re-enabled individually. Of course, if whoever created the extension is no longer supporting it, then you're pretty much screwed.
2. Click through the warning page and in the search box on the next page, put “xpinstall.signatures.required” and change the value from “true” to “false”.
3. Finally, restart Firefox and boom, the extensions should work again.
To be honest, I think automatically disabling unsigned extensions is a load of BS. It wouldn't have been a problem to me if Mozilla had included an option to re-enable them, but just unilaterally disabling something I chose to install and use is just plain being an asshole.