I've got this issue. When in firefox, i open a menu w/ a drop down menu & when i try to highlight the sub-menus-as soon as my pointer moves, the menu disappears. Sometimes restarting Firefox fixes it temporarily. I also notice that some of the drop down menus from the panel are initially invisible and only appear when mousing over them line by line. Not sure if the bugs are related. Using LMDE/MATE and FF11/12.
edit: I find that I can fix the FF dropdown menus (eg. folders in my bookmark toolbar) by clicking on a dropdown menu in the desktop panel, such as radiotray or dropbox. The issue is a recurring one.
Moved here by moderator (LMDE topic)