I clicked the Clean UP button on the Downloads window, and quit Firefox. When I restarted Firefox, it went from a 12+ bounce launch to a three-bounce launch on my iMac G5. The Mac Pro was even more surprising. It's brand new (one week this Monday), but I had imported info from my Powerbook G4. It went from five or six bounces to one.
Page loading speed seemed to be improved as well. I wish I could have tested it with a memory checker; maybe someone else could do this and see if it's improved.
[robg adds: I can't confirm this one, as my Downloads window in Firefox only ever has a couple items in it. If someone out there has a fully-loaded Downloads history and could do a bit of testing, please post your results in the comments.]

