hi vasilsi, please see the banner at the top of the support page - unfortunately there's a technical issue (failing addon signature verification) at the moment that's prohibiting the installation and in some case the usage of installed addons for firefox.

mozilla is working on a resolution, expect a fix soon. in the meantime you could temporarily try winding back your system clock by a day or so to get around the issue. sorry for the inconvenience...

Thank you.So it's a generall problem?How back my system clock?

yes, it is/was a general problem unfortunately. mozilla is starting to push out a hotfix now which should reach affected installations...

thank you.

At exactly the same time as add-ons disappeared, my tabs moved from below the url bar to above it. Is this part of the same bug, or a separate issue?