Ok, I somehow managed to remove this thing finally
This is what I've done yesterday:
Inspected the ad element and found from which website the ad content is getting loaded randomly.
Then I've done a file content search in all the drives for that web address using FileSeek program.
Came to know that it resides in the 'cache' of Firefox under Documents and settings/localsettings path.
Deleted all cache manually(which normally won't even if we uninstall Firefox).
Thanks for all your great support and concerns, I appreciate it.