Sounds like the issue could be Theme or Extension related.

Suggestion: Try launching Firefox in Safe Mode and see if the issue persists.

Safe Mode is a special Firefox mode that can be used to troubleshoot and fix problems. Safe Mode temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes) that might be causing problems. By comparing Firefox behavior in normal mode to its behavior in Safe Mode you may be able to pinpoint the cause of the problem.

How to launch Firefox in Safe Mode: https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode#w_how-to-start-firefox-in-safe-mode

If this issue is not present in Safe Mode, it's most likely because of an extension or theme.

• Switch to the DEFAULT theme: Menu > Add-ons > Appearance > Default
• Do NOT click the Reset button on the Safe Mode start window

Source: https://support.mozilla.org/en-US/questions/1082005

Sites may be able to remove the minimize button from a pop-up window. Does the problem affect new windows, too? You can press Ctrl+n to launch a new regular window, or Ctrl+Shift+p to launch a new private browsing window. If those have the normal buttons, you can drag the tab from the tab bar in the broken window onto the tab bar of a working window (assuming they're not all maximized...).