If you've got especially either Mac OS X 10.4 or Windows Vista, Mozilla QA would be very appreciative of and interested in any testing help you can provide for the Download Manager.
Bug 103487 has landed, which now means we *should* integrate with the various anti-virus vendors' software.
Additionally, we now have Vista Parental Controls (which can disable the download ability), and other Download Manager fixes/UI features left to land.
At present, there are ~ 250 either open or known fixed/resolved bugs.
It's daunting at first, but if you're interested in helping out, you can:
1) File/verify bugs
2) Write testcases (using Litmus)
3) Stop by in #qa of irc://irc.mozilla.org and we can help you test this and/or other components
Asking 'stephend' in #QA is the best pre-screen for Download Manager bugs, as I've probably run into most of them, or at least have read their bugs.
My testplan (work-in-progress) might get you thinking about ways in which you can help test.
Thanks in advance for your interest and help in making Firefox 3 great! (And yes, most of us hope that the "Clear / Clear All" button is returning :)
Check out these guys' amazing sets, if you like progressive trance and/or house...just click on "Downloads" using Firefox 2.0.0.6, and get what you can.