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.
Auto Tab Discard - Auto Tab Discard a lightweight extension that uses the native method (tabs.
Toby - Better Than Bookmarks
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
BrightExplorer - Add a tabbed interface to Windows Explorer.