i think i found the problem.
if i am running mozilla firebird, and i try to run mozilla, it simply starts a new instance of firebird. i think this happens with beonex and netscape too, but oddly not with galeon (ie whichever one is running, you just get more windows if you try to run another one). This might have something to do with the whole run-mozilla.sh business.
anyway, i can't open mozilla -mail while firebird is running since firebird has no mail client (since the relevant mail client is the standalone thunderbird program, not currently installed).
Thanks again for the prompt reply though.