The Mozilla Foundation has released Firefox 9 (it seems like just a few months age we were using Firefox 4...), and, with it, has fixed several memory safety bugs in the browser engine and in other programs, such as Thunderbird and SeaMonkey. These vulnerabilities were not critical, but, as the Mozilla Foundation's security advisory says, "we presume that with enough effort at least some of these could be exploited to run arbitrary code."
The Mozilla Foundation also released Firefox 3.6.25, with a single fix related to Java .jar files. This version of Firefox, for older versions of Mac OS X, is not seeing any more updates.
As usual, you can download a copy of Firefox, or launch your current copy and have it automatically update.