Linux Mint Forums

I am running LMDE MATE. In FF, when left-clicking at e.g. "Show more bookmarks" or the "NoScript icon", sometimes the actual sub-menu doesn't open.
If I then first shut down my running Thunderbird, the sub-menus in Firefox works again!
Why? What do they have in common that is disturbing and what can I do about it?

I don't think it is Thundebird that is interfering Firefox any longer.
The problem seems to stop when I turn off Compiz.