You have the kind of trojan that changes your home page on your firefox browser.
You ought to have noticed some sluggish behaviour on your computer as well, although newer computers are very fast and you might not notice.
If you have IE, see if this browser has a takeover of home page as well.
AVG found 3 trojans which is good, but the home page was already altered. Do not fault AVG because it is a free program. Perhaps the updates were not up to date (set this to be daily). I have seen terrible trojans on computers running Symantec. I use Antivir, another freeby with no problems for years.
First, you can run a 'free' online virus program from trend micro which gives you the perspective of another scan.... it also removes what is found.. This will not interfere with AVG which is good as you should only install one virus program on a computer at any given time.
http://housecall.trendmicro.com/uk
After this scan, try another 'free' download which is propular for these types of virus attacks.
http://download.cnet.com/Malwarebytes-Anti-Malware/3000-8022_4-10826546.html
Once you have used these 2 scanner programs, fix you home page on firefox and IE. Close all browsers and then open one to see if you are still being redirected to this strange browser page.
The true test is to reboot your system... if all is clear, you are good to go.
These types of trojans are not dangerous to you computer, but I would be wary of doing online banking. Never let windows save your passwords. I say this because you are going to sites that are questionable. It is spyware at it's worst. I have spent literally hours clearing these types of viruses from computers 'manually'. It involves shutting off running processes designed to take over your home page which was dropped on your computer when you entered a BAD site. It altered the registry to 'run' each time. Therefore it is no wonder it is still lurking on your computer as far as changing your home page.
Porn sites, hacker sites and sites that agree to including spyware for funding are to be avoided if you can.
good luck