That's unfortunate to hear. Possibly the website you were on captured your "Esc" key presses, the key that should allow you to exit the full screen mode. Have you tried switching to a different tab (Ctrl+Tab), and pressing Esc there?

Edit July 18, 2021: thank you HJ for explaining how you solved your issue, we hope no one else experiences it. The latest version should use the shortcut Shift+Cmd+I, it *previously* was Shift+Cmd+O (https://github.com/MarkTiedemann/fullscreen/commit/81f7c8a9701fe368bbe4acb4d638d330b667dbdd). You may have had an old version of the extension that caused issues.