What are some alternatives?
When comparing MySessions and Tab Mix Plus, 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.
Tree Style Tab - Tree Style Tab is an all-in-one Firefox extension that provides ability in order to operate tabs as a tree.
Session Buddy - Manage Your Browser Sessions
Kurator - Kurator is a Chrome plugin, that helps users to curate, organize and instantly access any link.