What are some alternatives?
When comparing Multi URL Opener and OneTab, 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.
Session Buddy - Manage Your Browser Sessions
Workona - A better way to work in the browser.
Tree Style Tab - Tree Style Tab is an all-in-one Firefox extension that provides ability in order to operate tabs as a tree.
Toby - Better Than Bookmarks