I believe that the Firefox code is tri-licensed. Mozilla, GPL and the more permissive LGPL. LGPL allows for "non-dirivative: work to be released without source. However, if I understand LGPL correctly, it defines such a program as one that links (i.e. a DLL) or uses the original code as library only.
I too am not a lawyer, but I would think that writing a browser based on the Firefox code would not apply as listed in the LGPL.
Very disappointing...
