Best answer by Bradforth
Welcome to the Webroot community.
Firefox is a good browser, but due to its size and customization it is prone to memory leaks, and unfortunately is well known for it. I would try doing a clean re-install of Firefox and see if that helps as it may be a minor plugin or addon causing your issue.
I used to have this issue a while back but since I did a clean boot of Firefox I haven't had any real issues.
Best of luck