Here is what has been happening for some time. If I open firefox while I have something like XMMS open, then at no point during the current firefox session can I do anything and have it play sound. It's like XMMS locks the soundcard, and nothing else can take it back. Turns out it's an issue with firefox. I changed two things, and it seems to be fixed now.

First thing, I added the following:

exec aoss /usr/libexec/mozilla-launcher "$@"

to the bottom of /usr/bin/firefox. The second thing, I added the following:

FIREFOX_DSP="aoss"

to /etc/firefox/firefoxrc. After this, the issue is resolved. Just putting it up here in case anyone else has the issue.