how can I fix internet virus for firefox?

You don't need to purchase anything - simply follow these instructions.

Download both of the following:

< Malware Bytes >
-->Download: http://www.download.com/Malwarebytes-Anti-Malware/3000-8022_4-10804572.html

< SuperAntiSpyware >
--> Download: http://www.download.com/SUPERAntiSpyware-Free-Edition/3000-8022_4-10523889.html

After downloading both, reboot your computer into safe mode, and run at least one full scan using each. Remove whatever it is they find.

Need help getting into safe mode?
--> http://www.bleepingcomputer.com/tutorials/tutorial61.html

I personally find spyware doctor (starter edition (free)) the best free anti - spyware program. It also includes 'inteliGaurd' that works in real time blocking virus from running.

See link below for the free working 'Starter edition' via Google pack.
(select option 1. on the list)

This will get the virus and help to make up the shortfall in security AVG has left you with. I would suggest moving to a better free anti-virus too. Avira is a good one.