No doubt Mozilla has already tested Firefox 3.5 heavily, but the simple fact of the matter is that you can never do enough testing.

"What we're really looking for is people to run through our security test case suite in Litmus,"Anthony Hughes of Mozilla QA wrote in a posting. "These tests have recently been updated and new tests have been created for Firefox 3.5."  

Another thing that always tends to happen is - though Firefox 3.5 has been in active development for a year, security researchers tend to come out of the woodwork once the release is out.

I do not see any 'smoking guns' though - as I did with Microsoft's IE 8 release. At PWN2OWN, a security researcher found a flaw in a pre-release version of IE 8. At the time of the official IE 8 release Microsoft stated that the released browser was not at risk. Yet several months later, IE 8 was patched for the flaw.