Sometimes the file that stores Window dimensions and positions becomes corrupted and contains nonsensical parameters. Since it is difficult to edit, you could rename the file to have Firefox use default settings.

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?

To turn missing toolbars on, you can use one of these methods to show the toolbar list and select the desired bars there:

• right-click a blank spot on the tab bar (or the "+" button)
• tap the Alt key to activate the classic menu bar > View > Toolbars
• in Customize mode > Show/Hide Toolbars (see: Customize Firefox controls, buttons and toolbars)

Back to normal?

Browser reopened with a much-smaller window size, resized almost to max, as of now button is visible. Will check it several times on later openings, and let you know.

Did you try to delete the xulstore.json in the Firefox profile folder?

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

I did successfully delete xulstore.json, but it only helped for about a day.

I refreshed FF tonight (7), and took care to set the bottom margin above the taskbar by a few millimeters to keep an eye on it.

<Reagan>Well, there you go again...</Reagan>

I opened a linked site from Twitter, and it was a new window.

Bottom scroll button: hidden.

Entered MOZ support URL in address bar, new tab.

Bottom scroll button: hidden.

Your turn...

Is the Firefox window maximized or in full screen or is the size less that the available screen width and height?

Full screen, not maxed. dL

Now it's even happening with the taskbar hidden, tho only when a new window is opened from Twitter!

Do you mean a new window generated by something in the Twitter page, or when you use Firefox's new window command (e.g., Ctrl+n or from the menu)?

Clicking a link in a tweet. dL

Dale_Latimer said

Clicking a link in a tweet. dL

Have you turned off the default behavior of diverting new windows to new tabs?

Sure did. dL

If I uncheck the box for "Open new windows in a new tab instead", load a Twitter feed and switch to full screen, then click a link, Firefox opens a new window in front of the full screen window. The new window is resizable. It seems that there may be enough room below it for the taskbar, but the Taskbar doesn't show because there is another Firefox window in fullscreen. Is that how it's supposed to work??

As far as I know, yes. dL

Having not heard anything on this in a while, I've turned the "open new windows in a new tab" back on, and have not had the issue since.

dL