When I switched from Windows to OSX, it bugged me a lot that my favourite browser had a serious problem on OSX: When I used the red X or a swipe gesture I had defined with “Better Touch Tool“, Firefox seemed to close but it didn’t reinstate all the tabs I had open when I restarted it. I found out the reason for this lies in the way that OSX handles closing of a program. It just closes the Window, but doesn’t close the program itself, I guess because it will restart faster if you activate it again then. Anyway, using the red X means to Firefox “close the window, but don’t close the program”, and therefore it doesn’t realize it needs to save the tabs for next start. Mozilla says this is no bug of Firefox, it’s just problematic how the Mac andles programs.
The workaround I use now is the Firefox Addon “Session Manager“. In it’s preferences, you can set “Close Firefox on close of the last Window” and “On close: save current session” and “On start: choose session: last session”. This let’s Firefox always open with the last session, that means with all the tabs you had open before you closed it. Happy Maccing, Mr Firefox. Also works with 4.0 btw.