I've been working on web extensions for multiple browsers lately.

Chrome and Firefox have workable ecosystems and accept more or less the same code, but package it differently.

Edge accepts almost the same code, but they had to change the "chrome" namespace to "browser". I suppose it was a question of pride. Also, Edge doesn't have a workable way to publish and to inline-install web extensions.

Safari accepts similar code to the rest, but organized differently. They have a slow and ponderous process for approving extensions for their store. They don't offer anything remotely resembling inline installation.