The missing bit of that is the actual profit. The praise of the world's hacker community does not pay off the mortgage. On the other hand, if it's open source, anyone can compile a client which doesn't show their ads, and give it away.

Don't get me wrong, I'd love Skype to be open sourced. But I can see why they haven't done so.

Actually, it looks like a lot of what you're referring to has been possible since fall of last year. See: http://developer.skype.com/ Skype now has an enhanced API and plugin architecture that allows exactly the type of firefox extensions that you're referring to. Unfortunately, the underlying communications protocol remains closed, and so requires a binary that appears to only work on 32-bit systems (not to mention ARM). I'm actually OK with them keeping the communications protocol closed-sourced, but they need to provide binaries for a greater variety of architectures if they really want this program to work.