To fix this : Find firefox.exe (right click the firefox icon on taskbar - if you have one - then right click again on the firefox icon in dialog) Firefox.exe > properities > compatibility tab > change high dpi settings > high dpi setting override > set to system This appears to fix the issue (for me anyway on latest win11 pro) I cant be the only one for whom this was a daily frustration - and couldn't find an answer on this forum. If someone knows another fix please let me know. Cheers.