- F1 (supposed to open MZX help, instead opens browser help)
- F3 (supposed to save, browsers make this do the same thing as Ctrl+F)
- Alt+C (text entry char select, Opera likes to block this one for no apparent reason)
- Alt+D (file manager delete, browsers like to make this highlight the current URL)
- Alt+N (file manager new directory, Opera likes to block this one for no apparent reason)
- Alt+R (file manager rename, Opera likes to block this one for no apparent reason)
- THERE ARE CERTAINLY MORE
- HONESTLY JUST BLOCK THEM ALL
I think it'd be fine to not do anything about F5 (play game/switch bomb type), since the browser function of F5 is actually useful, P can be used to start gameplay, and Insert can switch bomb types during gameplay.
F11 also may be useful and isn't used outside of the editor, though this adds a special behavior to escape and may generally be inferior to the MZX built-in fullscreen.
Something that might also be fine to leave alone is the Alt behavior of opening the browser menu when pressed by itself. I don't really have a strong feeling about that one either way.