I suspect that Mozilla being the primary developer and sponsor for many years actually meant that compatibility with all major platforms was prioritised; Mozilla obviously care about stuff working on Windows, and run lots of builds on Windows + I imagine a number of Firefox developers (if not drive) at least own a Windows machine for testing Windows-specific stuff!
I call out Windows because I think generally software people go for Mac > Linux > Windows (although Mac > Linux may be slowly changing due to liquid glass).