This could be caused by Google's CDN being blocked, and hence its jQuery not being available:
And somewhere in April 2011 this changed to:
Note that the above will not work if Stack Exchange's own
sstatic.net is blocked.
So, for future searches:
Stack Overflow works best with jQuery enabled
I don't see this in Firefox 3.6 (latest) on stackoverflow.com either.
I suspect you either have non-default settings, or a rogue plugin. Try starting Firefox in safe mode and see if it persists.
In my Firewall i had a rule to block the extension .js.I removed .js from the banned extensions list and added
to the exceptions list in blocked sites.Now everything is working fine.Thanks everyone especially Arjan.