This seems to be linked to the “Web Developer” add-on for Firefox, if you uninstall the add-on, the problem goes away. We have tested this with Web Developer 1.1.6 in FireFox 3.0.9. Other versions may not have this problem.

UPDATE :  I received an e-mail from the author of Web Developer : Chris Pederick.  It seems that although we found that removing the Web Developer add-on solved the issue, the fault actually lies with the HTML Validator add-on.    You will all be glad to know that an update for HTML Validator was pushed out this morning : Version 0.8.5.2.

HTML Validator Update 0.8.5.2

This is a statement from the author

- Previous versions hit a bug of FIREFOX 3.0.9 in DOM API ! The real solution is Firefox 3.0.10 but it will not be available before 1 month.  Practically, you do view source in Firefox 3.0.9 then crash ! Some lines of code that were working since Firefox 1.0 are now crashing Firefox… Due to this, I have to release very fast 0.856 to work-around the Firefox 3.0.9 bug So, the highlight of the lines with HTML errors is now disabled ! Unhapilly, I had no better solution.