What are some alternatives?
When comparing Multi URL Opener and Tab Mix Plus, you can also consider the following products
Session Box - Using websites with multiple accounts at the same time is made easy.
Tabs Outliner - Next Generation Session, Windows, Tabs Manager and a TooManyTabs Solution That Really Works.
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.
Session Buddy - Manage Your Browser Sessions
Tree Style Tab - Tree Style Tab is an all-in-one Firefox extension that provides ability in order to operate tabs as a tree.
Firefox Multi-account Containers - Sign in to two different accounts on the same site