Firefox uses unified builds, where a bunch of .cpp files are globbed together and compiled at once. That helps a lot, but a build still takes a bit of time unless you are on an absurdly fast machine. Chrome used to also support this, called "jumbo builds", but they didn't want to deal with the maintenance overhead. Presumably all of the Chrome developers employed by Google are using some kind of massive distributed build infrastructure so there's little impact of slower builds on individual developer productivity, so the use case of building on a single computer is not as prioritized.