Go to menu >> Options >> Tabs >> And unselect "Open new windows in tabs instead"

That didn't work.

there is no menu, option etc !

My problem is that suddenly, firefox overlays the current window, as well as opening a new tab when I right click and select open in a new tab. It happens more often, than not !

Make sure that you do not drag a tab slightly down in the browser window while clicking. Current Firefox versions have a feature called tear-off tabs. You can detach a tab from the current window and open it in a new window by dragging a tab in the browser window. You can drag that tab back to the tab bar in the original window to undo that detaching.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

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

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes