Firefox gets installed in addition to Firebird and as far as I could see, both worked fine. I did not try opening both at the same time and have since removed Firebird.
One small question: I had already downloaded Firefox independently of this package and it came with a new icon of a globe and fox. I could not find that icon in the version installed by the package and I just wonder what happened to it.
I did pacman -Sy firefox (on another test box, Syu offered to replace firebird with firefox) and it removed firebird and replaced it with firefox (when done through pacman).
Sarah if I get a chance I'll file the bug report tonight if no one else did--it wasn't a big deal, although firebird was removed the plugin was still there, so I just copied it over from the firebird to the firefox directory.