I have the following Extensions installed and enabled: (Tools->Add-ons)

20-20 3D Viewer 3.0.31.0
Adblock Plus 1.1.3 (using the EasyList (USA) subscription with a few additions)
Firebug 1.5.2
Flashblock 1.5.11.2
FlashGot 1.2.1.17
Java Console 6.0.04
Java Console 6.0.07
Java Console 6.0.13
Java Console 6.0.15
Java Console 6.0.17
Java Console 6.0.03
Java Quick Starter 1.0
Microsoft .NET Framework Assistant 1.1
Tab Mix Plus 0.3.8.2

I have the following Plugins installed and enabled:

20-20 3D Viewer 3.0.31.0
Adobe Acrobat 8.2.0.81
BitTorrent 1.0.0.1
DNA Plug-in 1.0.0.1
Google Earth Plugin 1.0.0.1
Google Update 1.2.183.17
Google Updater 2.4.1536.6592
iTunes Application Detector 1.0.1.1
Java Deployment Toolkit 6.0.170.4
Java(TM) Platform SE 6 U17
Microsoft DRM 9.0.0.4503
Microsoft DRM 9.0.0.4503
Mozilla Default Plug-in 1.0.0.15
PalmSource Package Installer 1.5.0.4
Picasa 3.0.0.0
QuickTime Plug-in 7.5.5
RealJukebox NS Plugin 1.0.3.69
RealPlayer Version Plugin 6.0.12.69
RealPlayer(tm) G2 LiveConnect-Enabled Plug-In (32-bit) 6.0.12.69
Shockwave Flash 10.0.45.2
Windows Media Player Plug-in Dynamic Link Library 3.0.2.629

There are many things about these lists that I don't understand, but there they are.

You might try updating Flash if you haven't already. Flash can be a real hog - I have FlashBlock enabled by default.

I doubt that you have a virus. I suspect that some extension or add-on in FF is giving you problems with some sites.

You might get a clue if you look at the FF status bar at the bottom of the FF window when it hangs. Many ad and graphics servers can time out or be very slow (http://www.boston.com is often very bad about that during the week). If you see a particular site being slow, you can add it to AdBlock so that it is skipped.

HTH a little. Good luck.

Cheers,
Scott.