I've wished Mozilla would do FF and TB
For the same reason why Breakpad, the current Mozilla crash reporter, replaced the one from Firefox 2 and older, Talkback. Talkback was proprietary software.
Windows Installer is also proprietary software, even though open source tools that can generate MSI packages exist. The current installer that Mozilla uses is essentially heavily-modified NSIS.
In the retail world there's a saying "the customer is king". In the software world is the saying now coming to be "the customer is the peasant"?
In open source, there is no such distinction between customers and developers. If one is unhappy with some aspect of the software, that person can simply code around it.
Mozilla software do use the GPL along with the LGPL and their own MPL as a tri-licence.
And Mozilla does provide a link to source code, though not in the most obvious way.
The universe does not have laws -- it has habits, and habits can be broken.
