I installed Firefox 3.0.9 because it fixes some security vulnerabilities. Next I got on with my usual snooping, i.e. View Source. Crash! No matter what site I looked at, it crashed. Luckily, someone had posted a solution at Google Groups and it is simple.
The problem lies with the HTML Validator add-on. It has been updated to version 0.8.5.6. Update your copy at https://addons.mozilla.org/en-US/firefox/addon/249