I don’t know what did you do, but I backup up all my extensions (copied the ~/.local/share/gnome-shell/extensions
folder), reinstall OS (Fedora 30), removed ~/.local/share/gnome-shell/extensions
folder, created a symlink to the backed up folder (link being in ~/.local/share/gnome-shell/extensions
. Then I’ve got the error.
I believe that the Firefox extension checks if ~/.local/share/gnome-shell/extesions
is a folder, but a symlink is not a folder (not even when it links to a folder), therefore a variable in the Firefox extension is left undefined.
Solution
The folder ~/.local/share/gnome-shell/extensions
must be a folder, not a symlink. In my case, I symlink ~/.local/share/gnome-shell
and it works again. If you don’t want/need to use a symlink, use regular folders and the issue disappears.