[im_not_1337]

Your not showing any indication of any viruses or anything malicious. You can remove the entires that show (no file) or (file missing) but thats just cleanup.

It may be something to do with your symantec software... Theres a couple of things you can try:
1) Updating windows at windowsupdate.microsoft.com/ in case of some sort of driver or corruption issue
2) Removing your Symantec software and see if the problem reoccurs
3) Completely uninstall firefox and all of your addons first, and seeing if the problem still exists with firefox.

Also, how often does this problem occur?

EDIT: What i meant in my first post was what the poster above just said, your looking for something else hogging resources that is stalling your browser. What he said you should do, watch the task manager (sorted by cpu and then sorted by memory) and see what happens when your problem occurs. Its likely that something will go to 99 percent cpu usage when the problem occurs, or memory will shoot up or something. Check and see what happens in task manager when the problem occurs.