I don't get it as well, what's the strategy behind that? They just reinforce Google's dominance so one of their main competitor just becomes stronger.
According to https://en.wikipedia.org/wiki/Firefox, it's released under MPL. But sure it's more restrictive than most code in Chromium.
As for whether there's a lot of MPL or a lot of BSD code probably doesn't have much practical effect for compliance. In order to comply, it's easier to publish all the code than to try to do the minimum for each component on a per-license basis.
MSHTML and EdgeHTML are also used as embedded components in non-browser windows software. (E.g. you can write UWP applications powered by JavaScript and rendering UI using EdgeHTML). Gecko supposedly doesn’t work well for embedding in that context.
Just look at how successful chromium has been in this space, stuff like Chromium Embedded framework, Electron, the myriad of chromium based web browsers etc...