What are some alternatives?
When comparing MySessions and Tree Style Tab, you can also consider the following products
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.
Tabs Outliner - Next Generation Session, Windows, Tabs Manager and a TooManyTabs Solution That Really Works.
Session Box - Using websites with multiple accounts at the same time is made easy.
Tab Mix Plus - Tab Mix Plus enhances Firefox's tab browsing capabilities.
Iridium - Iridium Browser is based on the Chromium code base. All modifications enhance the privacy of the user and make sure the most secure technologies are used.
Session Buddy - Manage Your Browser Sessions