instability

Help! My Firefox has become unstable -- for the past couple months it has been having this bad habit of locking up on me at random moments. I have tried defragging my hard drive and uninstalling and reinstalling Firefox, but I don't know what else to do. I have looked on their site for a FAQ addressing this but what I found hasn't helped so far, except that they recommend I run the program in Firefox Safe Mode. I'm doing that. So far so good...but is that a fix? Any suggestions???