Tabs Outliner is not available for Firefox but there are plenty of alternatives with similar functionality. The best Firefox alternative is OneTab, which is free. If that doesn't suit you, our users have ranked more than 50 alternatives to Tabs Outliner and many of them are available for Firefox so hopefully you can find a suitable replacement. Other interesting Firefox alternatives to Tabs Outliner are Tree Style Tab (Free, Open Source), Tab Session Manager (Free, Open Source), Sidebery (Free, Open Source) and Tab Mix Plus (Free, Open Source).
Warning: To those looking for a replacement for Tab Groups by Quicksaver (or some other similar tab grouping add-on), this add-on is not meant to be an equivalent replacement for it and differs considerably.
Organize your tabs into workspaces. Switch between workspaces to change which tabs are displayed at the moment. Features Each tab belongs to a workspace. New tabs are automatically added to the current workspace.