Firefox has some problems with certificates which might prevent extensions from being installed (at least on Android).

Announced here: https://discourse.mozilla.org/c/add-ons
Explained here: https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/
News about the issue: https://twitter.com/mozamo

Temporary solution:
1. go to about:config (enter in address bar of firefox), search for: "xpinstall.signatures.required", set it to "false", restart Firefox. Now you can install extensions again. Make sure to enable signatures again when the problem is fixed.