Prism is the shit.
Prism is. Really. It’s XUL app that uses Gecko to run webapps outside of your browser. You’re thinking: “But I already have FF. Why move something Outside of it.” Prism exists separate from your FF profile. When you start prism, you just start a wizard that lets you set up a new webapp. You choose the URL, what is displayed (location bar, etc), and how you launch it. Then you get an icon or shortcut or whatever that you stick in a convenient place.
I use it for my email at work, which is web-based. Firefox can run independently (crash or restart or whatever) without taking the email app out. “New Window” links open in an existing firefox, as per my desktop environment settings. It even pulls the icon from the favion for the shortcut.
Because it’s not starting up FF, it starts very fast. It maintains an independent cookie set, so you could make multiple prism apps for multiple accounts with the same webapp.