Workona - A better way to work in the browser.

Auto Tab Discard - Auto Tab Discard a lightweight extension that uses the native method (tabs.

OneTab - Whenever you find yourself with too many tabs, click the OneTab icon to convert all of your tabs into a list. When you need to access the tabs again, you can either restore them individually or all at once.

Firemin - Firemin will force Firefox to use less memory. It is safe to use, spyware free and completely free.

Session Buddy - Manage Your Browser Sessions

Dormancy - Dormancy 'retires' tabs that have gone unused for a while, freeing up that memory.