I'd love to be able to quickly switch between contexts within my browser, each with its own history, open tabs, (add-ons?), personas, etc. The idea would be to improve my personal productivity by, for example, having "Work", "Personal", and "Fun" workspaces set up. When I want to focus on work, I can just flip to my "Work" workspace that has all my tabs, bookmarks, etc for work. When I'm done for the day, I can just switch to my "Personal" workspace and do stuff there without disturbing the tabs and other stuff in my "Work" workspace, etc.
Creating new workspaces should be as simple as "Edit > New Workspace" or somesuch, so if I decide to start researching something (kitchen renovations, the history of cryptography, Christmas shopping), I can quickly and easily start a new workspace for that task. Optionally, allow workspaces to be password protected (which would be best for the Christmas shopping one).
You can emulate this idea in a way simply by using new profiles, but new profiles are a pain in the butt -- most people don't know how to use them, and you have to restart your browser, etc. People have also suggested bookmarking sets of tabs and flipping between them, but that really doesn't work at all.
Extra bonus points if workspaces can be synced via Weave.