My girlfriend recently got herself a shiny new MacBook Pro, and installed Firefox, as that’s her browser of preference. She kept having a problem that her tabs would not be saved when she closed Firefox, and asked me to look into it.
All the settings under Preferences were OK, so the problem was not with the settings. Likewise, the relevant settings under about:config were set correctly. We tried deleting prefs.js, but that didn’t help either.
Then, as a last-ditch attempt, I tried closing Firefox using the option on the menu bar. I then got the beloved question “Do you want Firefox to save your tabs”. I clicked yes, then restarted Firefox. All the tabs were reloaded. I then used the red X in the upper right-hand corner, the opened Firefox, and the tabs were not reloaded.
I registered the bug with Mozilla (bug #544880), and it seems like this is not so much a bug, as the way Firefox is handled in Mac OS X; when you use the red “x”-button, it does not shut down Firefox, but rather closes the window in question.