There's a few extensions that are known to cause a problem like that. You need to do a little troubleshooting to determine which one is causing your problem.

Do you have that problem when running in the Firefox SafeMode? http://support.mozilla.com/en-US/kb/Safe+Mode Don't select anything right now, just use "Continue in SafeMode."

If not, see this: http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+theme...

As I have read elsewhere, a recent extension OR add-on can cause problems. I started having the same problem of not being able to open more than one FF window. Checked for any updates to add-ons or new add-ons. Found that a Zonealarm toolbar 1.5.149.0 that loaded itself during an program update, caused the problem.