You might wanna try to disable signature check, see
blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/
Quote:
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For users, who run an old Firefox like version 52 ESR or on their Android device, a temporary fix could be, when you go to “about:config”, search for “xpinstall.signatures.required” and set it to False. But don’t forget to set it back to True, when your Firefox gets a real working fix.
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