Some comfort in not being alone in seeing a problem I guess ;)
The only thing that might be unique to my system is I installed:
javase6release1dp3.dmg
JavaEmbeddingPlugin0.9.5+f
I actually prefer using FF 1.5.0.4 for working with vBulletin because the full screen editor WYSIWYG is easier to use, and see what it will be. So that means often having both browsers open.
I have not touched Safari, no idea if this happens there - only when something else chooses to use Safari (rather than look and use the system default for WWW) does SAfari ever get launched.