Someone is going to take the chrome content API (or CEF or something similar) and make this happen and will become very popular (or even better, an "IDE" environment of sorts for browsing w/ good context switching). No, I am not talking about a separate window with hierarchical tabs, I am talking about being able to use some of this horizontal space I have plenty of.

The other barrier (which I guess PolyBrowser suffers from too) is that Chromium is so adamant on separate processes per tab that it becomes painful to use a lot of tabs. I wish they would officially support "--single-process" (doesn't need to be default or anything).

Edit: I see PolyBrowser is in XUL so it's already FF, pardon the comparison.

We're shooting to add the TreeStyleTabs addon in an upcoming release. PolyBrowser lets you zoom out to see all of your websites at once, so you might want to give it a try anyway.