It makes rather bad impression to correct numerous critical bugs related to security, and then to correct on short interval previous corrections.
The latest advisory description, especially the last sentence gives an impression that Firefox's code is not under tight control:
"One of the security fixes in Firefox 3.0.9 introduced a regression that caused some users to experience frequent crashes. Users of the HTML Validator add-on were particularly affected, but other users also experienced this crash in some situations. In analyzing this crash we discovered that it was due to memory corruption similar to cases that have been identified as security vulnerabilities in the past."