Just to elaborate on my last comment, when FF loses it window decoration and won't come to the foreground, it is the last active foreground window that continues to receive the keyboard input. If that happens to be something like a terminal or text editor, you can see it happening. If it's a window that wont accept keyboard input, you obviously wont. In both instances things like <ALT><SPACE> seem to work on that other window as usual. As soon as I minimise that window, FF seems to regain its foreground status and you can type in to in normally.
Just to elaborate on my last comment, when FF loses it window decoration and won't come to the foreground, it is the last active foreground window that continues to receive the keyboard input. If that happens to be something like a terminal or text editor, you can see it happening. If it's a window that wont accept keyboard input, you obviously wont. In both instances things like <ALT><SPACE> seem to work on that other window as usual. As soon as I minimise that window, FF seems to regain its foreground status and you can type in to in normally.