Hello,
The Mozilla foundation has messed up the signing certificate expiry for all Firefox web browser extensions and add-ons, thereby making them invalid during late Friday. It is a big issue right now and an urgent FF fix is under testing. Read more about it there:
https://discourse.mozilla.org/t/certificate-issue-causing-add-ons-to-be-disabled-or-fail-to-install/...
https://bugzilla.mozilla.org/show_bug.cgi?id=1548973
Yours Sincerely: Tamas Feher, Hungary.
The problem is now fixed: Mozilla foundation has released a new minor version (Firefox 66.0.4) for all platforms, which re-enables the extensions and add-ons, including F-Secure's.
Best Regards: Tamas Feher, Hungary.