Firefox is known for its add-ons. Firefox might consume lot of memory when you have opened lots of tabs and also installed too many add-ons. As a result your browser becomes very slow and it would be very annoying working on that. The best way yo free memory and to make firefox fast would be to restart it. Memory Restart is a firefox add-on that monitors the memory usage of the firefox and restart firefox if the memory usage reaches a certain threshold.
After installing Memory Restart add-on, restart your browser and you will see that firefox memory usage is shown in the status bar.
Once memory usage of firefox reaches a certain threshold, the status bar text display will turn red, and you can click on the status bar to quickly restart your browser.by default, maximum memory usage set by Memory Restart is 500mb which you can also configure it. First go to tools menu and select Add-ons. Then search for Memory Restart and click on options and set the value of memory you want to set so that firefox will automatically restart when memory usages reaches the value entered by you. This allows you to quickly restart firefox and fix any memory consumption issues, without losing your existing tabs.
Memory Restart add-on works with Firefox 3.6-4.0 versions.
Related – Decrease Firefox Memory Usage