I have discovered that Norton Internet Security (and Norton 360 as well) conflicts with Firefox version 30.0 and later. The culprit here seems to be the version 21 engine of both NIS and 360. (You can find out what version you're running by clicking on "Support" and then "About" on the product itself.) I run several Norton products on different machines with different versions, and so it was easy to establish a control group. I currently run NIS version 20 on three machines with no problem with Firefox. However, when I upgraded on two of the machines to the latest version of NIS (version 21) Firefox froze; indeed it locked up the entire computer. Fortunately, I also run Rollback Rx and so I roll backed my computer to the earlier version of NIS (version 20). Having done this, I then had no problems with the interface of NIS and Firefox. But, ah, my office machine, which was running Norton 360, updated automatically to version 21, and I was unable to roll it back to an earlier version. This too froze my machine with Firefox. I therefore installed Google Chrome and everything worked fine. In light of this, I am convinced that there is a conflict between NIS and Norton 360 (versions 21 and following) with Firefox 30 and later. The conflict, by the way, does not go away by disabling the Norton toolbar.
This has put me in a predicament: I have been a very long time user of Norton products, but how long can I keep running version 20 (even with signature updates)? I much prefer Firefox to Chrome simply because it is far more powerful. Any help to solve this dilemma would be greatly appreciated.