Try to open the system menu via "Alt+Space" and resize the window that way.

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox Title bar, especially if you have multiple windows or Private Browsing windows open to prevent losing tabs in unnoticed windows.

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

Try to open a new window and close that misbehaving window to see if that works better if you close and restart Firefox.

The 3 bar menu has no exit option the problem is not fixed

Does it still happen in Troubleshoot Mode?

Yes still happens in troubleshoot mode

You could try creating a new profile. In the article linked below there is a section telling you how to open the profile manager when Firefox is closed.

https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles

An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile.