Need help with computer problem..

In my old laptop whenever I open Mozilla firefox it opened at infinite number of tabs and kept going to a weird "My Web Search" engine thing... Is this a virus? I tried using my anti-virus but it didn't find anything... what is the problem?

http://support.mozilla.com/en-US/kb/Multiple%20tabs%20open%20when%20starting%20Firefox

Hopefully that helps a little.

MyWebSearch is known crapware/adware. You probably downloaded a shady program that installed the crap with it. Yes, there's even a MySearch adware toolbar for Firefox.

Not all AV programs detect adware. What AV program are you running? I don't recommend Norton/Symantec.

Also, what happens when you start Firefox in Safe Mode (NOT Windows Safe Mode)? Be sure to chrck the box to disable add-ons.
http://support.mozilla.com/en-US/kb/Safe%20Mode

Also be sure everything is up to date. Firefox 4.0 recently came out, and it's a manual upgrade if your running an older version. Even if upgrading to 4.0 seems to solve the problem, you may want to ditch your old AV program and use something like Microsoft Security Essentials:
http://www.microsoft.com/security_essentials/
and run another scan.

EDIT: 333rd post!

The antivirus I'm running is Avast and is constantly updated.. is this AV good enough or should I change it??

Avast is good, it has Anti-Spyware in addition to the AV.

Try starting Firefox in Safe Mode (see link in above post with directions) with Add-ons disabled, and post if anything happens back here.

Ok I understand