Works fine for me, I can simulate your problem by improperly closing Firefox.

The correct way would be to close the windows you don't want and then to exit Firefox through the File ("Alt+F") menu then use Exit or Quit depending on your system.

But I can simulate your problem by closing the good window with "X" in the upper right-corner first (the one with the app-tabs) and then close the other window by any means.

You restart Firefox and open a window, since you closed without app-tabs there are none so you just see a window with your home page.

Firefox has some things added to Firefox 4 and therefore in Firefox 5.

1. "Alt+S" (History menu)
2. "Restore Previous Session" (that's the window without the app-tabs, but you have to this first)
3. "Restore Recently Closed Windows" -- can choose which window to reopen based on name and the mouse-over tells how many tabs. or you just use "Restore All Windows" without the guessing.

I know you are on a Mac, and this affects Windows user more than anybody else, but it does affect other systems besides Windows, occasionally, perhaps more often now with the plugins container.

The following may not completely eliminate having to terminate Firefox through the Windows Control Panel but it will come very close.

Plug-in and tasks started by Firefox may continue after attempting to close Firefox The "X" in the upper right-hand corner closes the Window (same as Ctrl+Shift+W) but even if it is the last Firefox window, it does not necessarily close Firefox .

The only proper way to exit Firefox is to use Exit through the File menu, not the "X" in the upper right corner of last Firefox window. In the Firefox 4 and 5 that would be Alt+F then X

• Firefox hangs | Troubleshooting | Firefox Support
  http://support.mozilla.com/en-US/kb/Firefox%20hangs#w_hang-at-exit

Use the Windows Task Manger to remove all running firefox.exe in the "Processes" tab of the Windows Task Manager, then restart Firefox.

If Firefox will still not start, remove the parent.lock file from the profile which is created each time Firefox is started to prevent other Firefox tasks from running, see
http://kb.mozillazine.org/Profile_in_use#Remove_the_profile_lock_file

Avoiding Problems with close/restart choose either extension

Use to close and restart Firefox after enabling or disabling an extension, switching to a new theme, or modifying configuration files, then you don't have to worry about delay or have to look in the Task Manager to see if Firefox is closed yet.

Both extensions use the same keyboard shortcut "Ctrl+Alt+R" or a file menu option.

• "Restartless Restart" extension for Firefox 4.0+ only (2 KB download )
  https://addons.mozilla.org/firefox/addon/249342/
• For older versions use "QuickRestart" extension (34 KB download) (
  https://addons.mozilla.org/firefox/addon/3559/

Thanks so much for the quick response and indeed, you are absolutely correct about how it might have happened. I'm used to just quickly shutting down the whole system without closing separate windows. Since I never had anything that I wanted to keep intact on Firefox before, it didn't bother me to lose the page I was on when I closed. I like the idea of the App Tabs so I will close out from now on by choosing File and then closing the window properly. If it doesn't work I'll let you know. If it does -- thanks for your help.