I wish tabs were truly first-class citizens in the modern browser. I'd like to be able to select with shift and command, just like files, a subset of tabs to transfer to a new window, the ability to save and restore tab environments by name, perhaps even email an entire window of tabs to another user.
Chrom(e|ium) can do some of these things! You can use shift and control clicks just like in a file browser to select tabs, then drag the whole selection to another window, or off to the side to create a new one. Easily movable tabs + a tiling WM makes it surprisingly easy to keep dozens or hundreds of tabs open (very useful when you're trying to research multiple topics in depth at the same time).
It's my #1 most missed feature in Firefox, where you can move tabs individually between windows, but not in a group. Trying to split off a group of related tabs into its own window is so painful that I often just kill them all and manually navigate back to them in the new window. It's a shame that they wasted so much effort on the Tab Group feature just to abandon it, instead of adding a tiny bit of functionality to the existing tab interface.
AFAICT, the Chrome multiple tab drag to separate into a new window doesn't work on Mac OS X, only on Windows (and Linux?). This one really hurts me every time I need it on the Mac.
What's even more frustrating is that multiple tab select works (at least, UI-wise it appears to), but then when you actually drag the tabs out only one comes along.
I can do all those things in Firefox with the extensions Tree Style Tabs, Tab Mix Plus and Multiple Tab Handler. Hard to say which extension does what.
Using the "Windows" sidebar of Opera 12 and its session management you can literally do all of those things. It could do these years ago out of the box, no plugins needed.
