Screenshot is attached now

hi, i cannot reproduce that behaviour on a non-hidpi-device. do you get the same problem when you launch firefox in safe mode once for testing purposes?

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Tried it with disabled Addons and Safe mode as well. Still getting same.

Several Firefox 39 users who updated to Windows 10 have reported that the minimize, restore/maximize, and close buttons are not sized correctly with Firefox 39. Hopefully this will be fixed in Firefox 40, which has numerous compatibility tweaks for Windows 10.

Until then, you could try Ctrl+t to launch a new tab, or possibly use the Customize feature to stick another icon between the "+" button and the tab list button. Then I suspect the minimize button would overlaps the sacrificial button instead of the new tab button.

I have attached a sample screen shot as an example (Windows 7, though), and this article describes the Customize feature in more detail if you haven't used it: Customize Firefox controls, buttons and toolbars.