One of the main purposes was to make developers lives easier so you can have a single codebase instead of juggling with browser specific APIs and configs. Otherwise many developers would just go with a market default.
You can generally have a common background.js file, maybe with a few conditionals, but the manifest.json file needs to be different for Chrome and Firefox.
I decided to keep two foo-manifest.json versions in a manifest/ subdirectory, and copy one to ../manifest.json depending on which browser I'm testing. It's a bit tedious. Not sure if a better solution exists.
It's a work in progress: https://bugzilla.mozilla.org/show_bug.cgi?id=1609923
> I decided to keep two foo-manifest.json versions in a manifest/ subdirectory, and copy one to ../manifest.json depending on which browser I'm testing. It's a bit tedious. Not sure if a better solution exists.
Personally, I favor having a build step that produces the final manifest. The basic idea is that you have a common manifest.json + paired down browser-specific files that override the general settings as necessary. It's not a perfect solution, but I think it makes maintenance a bit easier.