Mozilla Labs » Prism
Mozilla Labs » Blog Archive » Prism is workable. Honestly all it does is to install another — modified — Firefox on your computer and to launch whatever URL in a separate window. But it’s great if you have to keep track of multiple gmail accounts. No need to log in and out or switching between Firefox and IE.

Right now it does not support any plugins, so my beloved RememberTheMilk function is gone. Too bad.
Unless they have a way to handle offline data, I am not very enthused about it.