Tree style tabs[1] have changed my life. There are plenty of times I get near 100 tabs. It automatically opens them as subtabs when ctrl or middle clicked. This is great when browsing source trees online via github, google code, etc.
I combine "Tree Style Tabs" with the Tab Group[1] feature (added in FF4 I believe) instead of using a separate window for each class of tabs that I'm viewing. Those 2 combined with the "Pin as App Tab" option for Gmail and a couple of my control panels makes for a pretty efficient setup usability wise.
I really would love to see memory efficiency improve. I restarted FF yesterday and I'm already at 2.22 GB. It would be great to be able to set a ceiling on the amount of RAM FF is able to get its greedy little hands on. My iMac has 16GB, and I've seen FF reach half of that before when running a big Flash based web app with a hundred or so other tabs open. My little MacBook Air only has 4GB, so being able to tell FF to grab no more than 1GB or so would be fantastic!
Vertical tab management is far superior when managing a bunch of tabs. I am not sure how anyone who has more than a handful of tabs can get by horizontally, not sure how people can tell tabs apart when only seeing the favicon & a few characters of the title. What with widescreen displays being the norm now it just makes sense to do vertical tabs.
[1] https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...