wtf is compiling, by the way?
The problem with pre-compiled binaries is that they are distribution specific, they assume you have certain versions of libraries that come with specific versions of an OS.
When you compile programs, they often come with configure scripts that search your machine to see what versions of the libraries you have and where they are located, so when it compiles the code it links to the proper files and you can run on your personal machine's configuration.
When they say Mozilla is open-source, they mean that the actual code that makes up the program is available on the Web, so you could change it and have a custom browser, for instance.
