In FireFox, if you press the ' key (next to enter) in the Quick Reply box, it brings up the "Find" box.
It does it randomly in the advanced reply box too, just now for instance, but as soon as I copy/pasted the ' from notepad, it acted normally.

Don't know how it's confusing it with CTRL+F / F3

It's not me (I don't think), because it only does it on this board.