Mozilla switched its Mac builds from gcc 4.2 (or rather the Apple fork of it) to clang, because the cost of doing the switch was lower than the cost of avoiding all the bugs in said fork of gcc 4.2 that didn't exist in the other three compilers the codebase also had to compile with (gcc 4.4 on Linux and MSVC).
Mozilla switched its Mac builds from gcc 4.2 (or rather the Apple fork of it) to clang, because the cost of doing the switch was lower than the cost of avoiding all the bugs in said fork of gcc 4.2 that didn't exist in the other three compilers the codebase also had to compile with (gcc 4.4 on Linux and MSVC).