It appears as if Firefox has some problems with Addon installation
and activation. If your Firefox is Version 66.0.3 and older, you’ll find
most if not all of your addons being deactivated. Reason for this is
an expired SSL-certificate that is used by Firefox and mozilla.

There are several workarounds to get the addons back to working state but it needs a bit of knowledge to do so. One is to implement some piece of script so that Firefox activates the plugins without SSL check. But this can become a security issue and a backdoor for non-authenticated plugins that may contain also malware.

The second workaround is to use Firefox ESR 60.0.2.

Update May 6th, 2019 – 09:00:

The problems have been completely solved with Firefox 66.0.4 hotfix. Addons will now install and load fine again.

If you haven’t updated your browser, do so now to receive full addon functionality again.