You may have a corrupt xulstore.json file. https://support.mozilla.org/en-US/kb/changes-toolbars-and-window-sizes-are-not-saved

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

What is your computer system and Firefox?

I assume Keypass is an add-on, so in order to regain access to Firefox without it, you could try starting Firefox in its Safe Mode. That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Can you get to your home page?

If Firefox is stuck in "full screen" view, then the F11 key should switch it back to normal view (or vice versa). If Firefox is in normal view but the window is just way too big, please see Fred McD's suggestion.

Finally, if Firefox is distorting the screen resolution of your system while you are using it, make sure you have not set backwards compatibility options in the shortcut you use to start Firefox. Right-click the shortcut > Properties, Compatibility. On that tab, uncheck boxes and deselect old versions to make sure Firefox uses the full capabilities of Windows 7.