Try the Firefox SafeMode and see what happens. https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

Try to rename/remove the xulstore.json file in the Firefox profile folder with Firefox closed.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

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

the-edmeister said

Try the Firefox SafeMode and see what happens. https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

I refreshed and did the SafeMode thing - still no sizing button. Now the minimize button is gone, too ...

What's next?

Are you possibly using Windows in tablet mode instead of desktop mode?

Good hypothesis, but no.