Firefox has a fair number of bugs, but before they can be fixed, they need to be triaged. How about going through the New bugs and confirming the ones you can reproduce? And for the confirmed ones, check that it's still reproducible. In both cases, add any necessary information you can. For the New bugs, ask for necessary information too.
The Mozilla Team has a Triagers' Handbook with all the information you need to help triage Firefox bugs. So go check out the Firefox 3 Hug Day bug list. If you've got some free time, why not help triage these bugs?