Why?
I had been keeping until a few days ago a separate firefox window just for gmail. But I’m tired of Firefox freaking out and deciding that it only knows the email address that I don’t want to use for a contact that has two emails.
When I switched to using Safari, I still wanted a separate window, because Safari likes to open tab sets over the current tabs that are open (which makes me lose my gmail window). Also it’s tough (impossible?) to set up Spaces.app so that my safari gmail window is always in a separate space. When they are both in the same space it is awkward to get from one window to another…
Now I can assign Gmail Browser to its own window and use cmd-tab to navigate between them!
Plus that page showed me where the nightly builds of webkit are, Double Word Score!

