Firefox is released under the Mozilla Public Licence 2.0, a Free and Open Source licence approved by both the FSF and the OSI. Both those organisations require that in order to be approved, a licence must not forbid any particular kind of use. The FSF calls this Freedom 0, The freedom to run the program as you wish, for any purpose, and the OSI calls it Criterion 6, No Discrimination Against Fields of Endeavor. [0][1]
Is Mozilla's position that Firefox is actually subject to the union of the MPL2.0 and their other terms? If so, that disqualifies it as Free and Open Source software according to the usual definitions.
edit I see I'm not the first to point this out on HN. [2]
[0] https://www.gnu.org/philosophy/free-sw.html#four-freedoms