After upgrade to 5.0.9, it looks like the javascript keystroke code is not over-riding Firefox's (3) keystrokes... for example, in the HTML editor (create new mail window):
Ctrl-B no longer does bold, it switches from HTML to plain-text mode
Ctrl-I no longer does italic, it brings up the sidebar (History)
Ctrl-U no longer does underline, it shows the HTML source
Alt-S no longer sends, it does the History drop-down (Ctrl-S still works)

Zimbra 5.0.9 NE RHELx86_64, FF 3.0.1