Holy Smokes!
I found a solution. If I use the onclick event instead of onmousedown, it works perfectly. I don't know why there is a difference, or why the difference only affected my firefox.
I can't remember why I was using onmousedown in the first place, but there was a reason at one time. I'll have to go back and try onclick in the actual application and see if there are any other issues.
Thanks for the help along the way.