Do you get a thumbnail or other indication on the Windows Taskbar that there is a Firefox window "somewhere" but it isn't visible anywhere?

If there is a thumbnail (or a window title listed above the Firefox button on the Task bar):

Sometimes the Firefox window opens off-screen. If you have a multiple monitor setup, make sure both monitors are on. Other users have reported that sometimes Firefox behaves strangely when the second monitor is off. You already indicated that both monitors are on in your case.

In all other cases: You often can force the Firefox window to appear on the screen by right-clicking the little thumbnail image above the Taskbar and choosing Maximize. Does that work?

One possible cause for this is that the file which stores window sizes and positions has become corrupted.

Method #1: If you can get a maximized window:

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Show Folder" button. This should launch a new window listing various files and folders in Windows Explorer.

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button
• (menu bar) File > Exit

Pause while Firefox finishes its cleanup, then rename xulstore.json to something like xulstore.old. If you see a file named localstore.rdf, rename that one to localstore.old as well.

Start Firefox back up again. Do windows display normally again?

Method #2: If you cannot get a Firefox window at all:

Shut down Firefox by right-clicking the taskbar icon > Close All Windows.

Using the Run dialog (windows key + R) or the Start menu search bar (or in Windows 8 maybe the charms bar search bar?) type or paste the following and press Enter to drill down to the Profiles folder:

%APPDATA%\Mozilla\Firefox\Profiles

Here you may see one folder -- in which case, double-click into that -- or more than one folder -- in which case, double-click into what looks like the most recently updated.

Scroll down and rename xulstore.json to something like xulstore.old. If you see a file named localstore.rdf, rename that one to localstore.old as well.

Start Firefox back up again. Do windows display normally again?

Thank you jscher2000, it worked! I did have the thumbnail on the task bar that indicated Firefox was open. When I right clicked it "Maximize" was grayed out. I went to Method #2 and that was successful. Firefox now appears on my primary screen and seems fully functional.

Thanks so much for your help.

Jerry