It's actually the very same main.dart.js (dart2js compiler does not have ability to target a specific browser, its output is supposed to work in every supported

It's actually the very same main.dart.js (dart2js compiler does not have ability to target a specific browser, its output is supposed to work in every supported browser). 359kb is its compressed size, 990k is its uncompressed size. Firefox shows compressed size in the "Transferred" column and uncompressed size in "Size" column. In Chrome by default you see compressed size only, but if you click "Use large request rows" then you will see both compressed and uncompressed size.