At CyberNet Technology News, they have a nice article about Firefox’s memory usage and what extensions you can get rid of to lighten the memory load. Some of the memory leakers are: IE Tab, Session Saver, and Forecast Fox.
Memory leaks will slow down Firefox’s performance. However, if you don’t see a problem and you like browsing the way you are browsing, just keep doing what your doing. The sky isn’t falling just yet, but it is good to have this information handy.