I know we already have Portable Firefox, so my idea may seem stupid, but hear me out. One of the features I saw requested for PFF was native support for multiple profiles. So, I'm thinking of answering that request by writing a brand-new launcher that allows the user to select a profile on startup. As many profiles as necessarily can be created, and they'd be stored as subdirectories of the PFF profiles directory (or whatever my equivalent was).
I'll probably make this one way or the other, but I'd like to know if anyone would be interested in it. It would be completely open-source, just like John's launcher. However, instead of NSIS, I'd write it in C++ (sorry, I can't stand NSIS).