Please give us more information

Not sure what else I could give you - it's all in my first post.

Sometimes a window gets "off screen" somehow. You often can force it 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.

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.x 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.

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

That worked like a charm! (#2) Thank you!

Now, do I need to do anything with the two files I renamed? Or just leave them with the .old extensions?

I suspect you won't need those files. Maybe give it a day or two before getting rid of them, in case you notice anything else new and strange with window/dialog sizes/positions.

None of the solutions here worked. It seems that mozilla fucked something seriously up with 17.08.2015 update

Hi Estlib, this thread is about making a window that you can see is open on the Taskbar, visible. Your issue is different because you never even get a window at all. So let's continue with your problem in your thread here: https://support.mozilla.org/questions/1078544