5th
Using Camino as a site-specific browser for Google services.
I had been using Fluid.app in GMail mode, but email address auto-complete stopped working.
Also, Fluid browsers use the same cookie store as Safari, and Mail, and every other WebKit instance—integration is a blessing and a curse—which means they aren’t actually as isolated from each other as I thought. (They have different javascript processes and session stores, which makes cross-site-scripting less likely, but any application that performs auto-login on receipt of a stored cookie is still potentially scriptable.)
Camino is Firefox under the hood, but developed as a native Macintosh app. More importantly, it has it’s own cookie and certificate store. As a bonus, it’s open source. And it doesn’t have any annoying GMail bugs.
The only downside: there is no “open domains outside of google.com in default browser” option. I can live with that. At least when I go to other domains I won’t be logged in already.