Hi orlando_au, This is a known issue that can be fixed by resetting Firefox's window state. Here's how to solve it: First, completely quit Firefox: Click on Firefox in the Dock Press ⌘ + Q or select "Quit Firefox" from the menu bar Then try this fix: Open Finder Press ⌘ + Shift + G Enter: ~/Library/Mozilla/Firefox/Profiles/ Find your profile folder (it ends in .default) Delete the file called windows.json Restart Firefox If that doesn't work, you can also try: Holding down the Option key while launching Firefox to reset window positions Checking if Firefox is running in full screen mode by pressing ⌘ + Ctrl + F Let me know if you still have issues after trying these steps!