Hmm, if it's a movable window, that position commonly is set by the website based on what it understands to be your screen size, or close to the "last used" position. If it's a JavaScript prompt (for example, a one-line input control with OK and Cancel button), Firefox usually centers those vertically and horizontally based on its own computations.

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?

Thanks for the reply, but it turns out my issue was caused by a much simpler problem. The window size when not maximized had been changed and moved to the bottom of the screen. Once I deselected maximize and then corrected the smaller window size and position, the pop-ups went back to opening where they should.