A vulnerability has been discovered by security experts in the Firefox 3 browser which can cause a DOS. At the minute it's only a theory that it could also execute arbitrary code, but considering it can cause the browser to crash it is a possibility. The issue comes about with XSLT causing a stack overflow during the transformation of XML. At the time of writing this has not yet been patched (Firefox 3.0.7) but unless code execution is proven it's not that big a risk or problem.
UPDATE: Mozilla have now released Firefox 3.0.8 which fixes this vulnerability, so be sure to update!









