The state space is too large for these algorithms to be effective on Firefox as a whole, and there are many libraries we just don't care about when browser fuzzing.
eg. if AFL/libFuzzer manages to hit a path that makes an input appear as gz encoded, the "novel" zlib coverage is very attractive to the algorithm, but that's a very inefficient way to fuzz zlib.
Most of these libraries are targeted specifically by OSS-Fuzz [0] and their integration into Firefox is fuzzed with libFuzzer using the fuzzing interface andrei mentioned.