Hi JAC@Mozilla,

I'd try starting Firefox in Diagnose Firefox issues using Troubleshoot Mode. If you don't have the issue while all of your add-ons, extensions, and themes are disabled, you can try adding them back in one by one until you find the culprit. You should look at the Extensions and Themes troubleshooting guide and the Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems article as well.

You could also try Creating a new profile.

Hopefully this helps!

I found a workaround that solved it, though I still don't know the cause.

Right clicking the Firefox icon in taskbar doesn't give the option to maximize the window, but I realized that holding the mouse over it, I get a preview of the window (empty, shows only menubar). Right clicking on this preview, I do get maximize. And now I CAN see the window.

But there is still a bug somewhere. If I click "restore down" the window disappears again.

The workaround I found was this: - Maximize the first window (with 8 tabs). - Open a second window. - Pull all tabs to second window. - When the last tab is pulled, the first window is automatically closed and removed.

The new window seems to work normal. (I'm writing this...)

You can also check for problems with the localstore.rdf file if it happens again.

• http://kb.mozillazine.org/Corrupt_localstore.rdf
• https://support.mozilla.org/kb/Toolbar+keeps+resetting