There are things besides memory leaks that can cause this that can be difficult to track down. Keep in mind that I am running FF3.6, so I do not know if these still apply in the latest version. If any of these issues apply to you, and you also have an issue with a memory leak, the problem could be made much worse than if it was just a memory leak. It's worth it to at least considering the possibilities.
There are some add-ons (and settings in certain add-ons) that save enormous amounts of data to the places.sqlite database.
Disabling the add-ons doesn't remove it, so it won't fix the problem if this is what is causing it. First you have to identify which add-on is doing it, which can be very difficult and will require a fair amount of web research in many cases. Then you have to create a new profile and not use that add-on or disable the setting in it that causes the issue. If you just create a new profile without tracking down the add-on causing it, the problem may go away, but return a few weeks or months later.
For example, the Google toolbar has a start page setting that causes it to save a screenshot thumbnail of every page you visit to the places.sqlite database. This database can easily grow to over 1 gig in size in a few weeks if you are visiting a lot of pages. You have to disable that start page to prevent issues.
There is another issue that can cause the intermittent freezing and unresponsiveness, also related to the places.sqlite database, and that is empty space causing it to become fragmented. Vacuum Places Improved
is a great little add-on that will automatically take care of that. I have not had a problem with this issue since installing it. I have it set to automatically clean after every 25 starts.