Comment 2 for bug 1402059

This is still a problem in Firefox 44 on a stock install of 15.10. When Unity's menu bar is opened, Firefox's menu bar is opened as well, but it only shows up as a two pixel-tall line. If you go to the customize screen the thin line is visible there; you can even add icons and whatnot to it.

This probably hasn't gotten the attention it deserves because older Firefox profiles may not be affected. Well, those two lines of pixels are lost but the tabs don't suddenly shift down so you don't notice that anything is wrong (I didn't realize the issue until I reset my Firefox profile). I'm not sure what version of Firefox changed this behavior.

Regardless, I can confirm Carol Cannas' fix works great.