There is an article about this issue, which says, that this is due to an expired certificate:
The workarounds stated there, do not work for my extensions, so i have to wait, that firefox gets a solution (and a new certificate).
For all others with this problem: Do not deinstall your extensions (like i did) - just wait...
Edit: Here the statement from mozilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1548973
On twitter i found this nice comment
Edit: For latest infos read https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/