Mozilla Stops Serving Firefox Security Updates
Firefox released up-to-date versions of Firefox earlier this month when it patched 15 vulnerabilities, 11 of them labeled "critical." However, Mozilla stopped offering Firefox 3.5.12 and 3.6.9 due to a bug that crashed some users' machines after they'd updated, then restarted the browser:
We've limited updates to Firefox 3.6.9 and Firefox 3.5.12 at this time as we evaluate some early feedback which indicates that a subset of our user base may be finding the releases unstable.
The article notes that users wishing to upgrade older Firefox versions to 3.5.12 or 3.6.9 can download them from Mozilla's website, then install them manually over existing copies.