Have you tried using a different browser? Download Chrome here: https://www.google.com/intl/en/chrome/browser/
See if you have the same problem with the same websites. Chrome has flash built into it, so if it works, it may be a problem with flash for Firefox. Using Firefox, go here: http://get2.adobe.com/flashplayer/
Make sure you uncheck the McAfee Security Scan option... then download and install it. NOTE: You'll have to close firefox before installation for the installation to work properly.
If Chrome freezes too, then the problem may lie within Java... You said these are up to date, but they push updates frequently. The most current Java version as of the timestamp of this post is Java Version 7 Update 17. You can get it here: http://java.com/en/download/windows_xpi.jsp?locale=en
If chrome didn't freeze, but Firefox still does after installing the newest Flash and Java, then next step is to disable your add-ons.To view your add-ons in firefox, click the orange Firefox button in the top left of Firefox, then click Add-ons. Click Extensions from the left menu. Remove all extensions there. Next, go to Plugins in the left menu. You want to keep Adobe Acrobat, Java Deployment Toolkit, and Java Platform enabled. You may see a Shockwave Flash plugin-there as well, but disable it for now. (NOTE: Shockwave isn't too popular, but updating it may help as well: http://get.adobe.com/shockwave/
Restart Firefox after you've disabled the Add-ons and test to make sure the sites work properly now.