Firefox isn’t very modular. There’s just a C++ part and a JS part and they can be built independently. My understanding is that new developers are encouraged to focus on the JS. The devtools team is entirely JS, with some fancy modern affordances (TypeScript type definitions!) that teams working on the older parts of the JS codebase don’t have.
The build process for the rest of Firefox could be as modular as devtools. When I was involved with Mozilla (and for a while after), it was a frequent criticism of mine that no one was prioritizing making the contribution process as easy as possible for people who didn't have an email address ending in @mozilla.com.
Zotero has (had?) a really cool build system. Zotero is written on an Electron-like architecture, except it uses a Gecko-based runtime instead of Blink. (Firefox is the same way and has always been this way, and this predates not just Electron, but Chrome/Chromium, too.) Similar to the way that Electron apps' build scripts generally work by downloading a prebuilt Electron (rather than requiring developers build it from source), Zotero works by downloading a release build of Firefox, unarchiving everything, sweeping away all the XHTML/XUL/CSS/JS that comprises the Firefox UI and application logic, and then swapping in the components that make up Zotero. (In other words, just how it should work 90+% of contributors who want to submit patches against Firefox nightlies, too.)
