Hi there.

That is not native behaviour - Firefox should still use the standard Ctrl + F combo. Try pressing Ctrl, Alt and Shift on your keyboard to release any sticking modifier keys (they sometimes get stuck in software if not physically).

In case it still won't work, I'm trying to find an alternative for Customizable Shortcuts. It used to allow you to set your own hotkeys, but it's been pulled from the add-on database.

This might be the best alternative: https://addons.mozilla.org/en-US/firefox/addon/keybinder/. It is apparently based on some of the same code as the now-defunct Customizable Shortcuts extension. This should allow you to check and change your shortcut keys - which Firefox does not do natively.