What are some alternatives?
When comparing Open Tabs Next to Current and Tabs Outliner, you can also consider the following products
Session Box - Using websites with multiple accounts at the same time is made easy.
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.
Tab Mix Plus - Tab Mix Plus enhances Firefox's tab browsing capabilities.
Session Buddy - Manage Your Browser Sessions
Tab Utilities - Tab Utilities is an ultimate tab solution for all kind of users who want to manage their tab, create multiple session and search for new tabs, etc.
Tree Style Tab - Tree Style Tab is an all-in-one Firefox extension that provides ability in order to operate tabs as a tree.