Thank you for your review! :) The signature issue is an internal Firefox issue that is affecting all extensions. Mozilla is actively working into fixing the problem. You can follow their progress on the dedicated forum thread (https://discourse.mozilla.org/t/certificate-issue-causing-add-ons-to-be-disabled-or-fail-to-install/39047) and on the bug report (https://bugzilla.mozilla.org/show_bug.cgi?id=1548973).
EDIT: The problem has been fixed with the release of Firefox 66.0.5. Please see the dedicated blog post (https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/) for more details or if you are using an older version (52 to 56) of Firefox.