VLC even ships with everything to "crack" DVDs. Those tools are flying under the radar, but Firefox probably won't (with two direct competitors being part of MPEG-LA).

Firefox makes money (through the search engine integration at least), VLC et al do not.

It might also help that VLC is a french project. Laws and patents are sufficiently different that it takes effort to build a case, while for US cases the MPEG-LA + members have probably a template on file where they only have to fill in the victim.

They are, but redistributing them in certain jurisdictions may be illegal, which prevents many from actually exercising the rights the license grants them. For example, until not long ago, Debian would disable those codecs in their ffmpeg package.

Firefox can't afford to play fast-and-loose with patent liability.