Comment 9 for bug 917753

David, for the avoidance of doubt, the way you propose Firefox should work is indeed the way it used to work and the way it still does work if you get rid of Unity. See my bug report bug #1004994, submitted in May (when I was unaware of your bug report), which today has been marked as a duplicate of this bug. It's still well worth reading bug#1004994, because it makes it very clear that the cause of the problem is a lack of functionality in globalmenu-extension.

As I understand it (and unfortunately I don't know how to put this tactfully), the author of globalmenu-extension is saying that it's not his problem because other parts of Unity don't offer the facility he needs, yet he's not prepared to take ownership of the problem and pressure those other designers/developers to provide what he needs.

And so this bug has languished for 7 months. Perhaps that's because those who routinely use the mouse feel that this issue is a rather academic one. But for those of us who keep our hands on the keyboard almost all of the time, and who switch to other Linux versions and even other operating systems, but still consider Firefox the best browser on all of them, it's a damnable irritation that Unity is the one place this doesn't work. Indeed for those who are disabled in some way that prevents use of a mouse I imagine this breakage is infuriating.

As a workaround you could try going into Firefox menu Tools | Add-ons and disabling globalmenu-extension. That worked for me on 12.04, and worked for a while on 12.10 alpha, but then other issues a month or two back prevented me from running 12.10's updates, so I can't say if that's still true. Of course, disabling globalmenu-extension will reverse some of the menu placement that Unity tries to rejig.

Or, of course, you could throw out Unity and choose one of the other desktops.