Yep! Firefox supports vertical sidebars (which Chrome doesn't I guess?), so you find a lot of Firefox add-ons for vertical tree-style tab sidebars.

Sidebery is my favorite by far, because it also lets you organize the tabs in "panels", and lets you set keyboard shortcuts for navigating across panels. So you don't end up with a giant list of 50 tabs.

Eg I have 1 panel for email/chat/music, and a separate panel for each task I'm working on. Makes things a lot more manageable, since each panel usually has only 5-10 tabs.