This is a problem on the server.
However it might get triggered by Firefox sending corrupted or invalid data.

Clear the cache and cookies only from websites that cause problems.

"Clear the Cache":

• Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox/Tools > Options > Privacy > Cookies: "Show Cookies"

thanks for your reply cor-el...

This happens after paying my bsnl broadband bill online and clicking the print button for the receipt.

So, i'll try your solution next month cause i already paid this month bill. I'll try it on the new version 26 or hopefully 27 and check if this solved.

I'll also notify the bsnl about this server error but knowing them they will just blame it on my system and firefox and tell me to use IE (Which is just there cause it comes with Windows and required to open some government sites).

Hoping for the best...Thanks again..

cor-el is right. However, if they do blame your system, try troubleshooting the extensions that are installed on the browser by starting it in safe mode or disabling them for process of elimination.