I don't know exactly what the the person you're replying to had in mind, but support for different parts of the manifest varies (especially in Manifest V3). While it's possible to write a single manifest that works in all browsers (with warnings), doing so requires more than a little specialized knowledge.

For example, Firefox does not currently support the `optional_host_permissions` top level manifest key. To work around this, developers can declare their optional host permissions in both the `optional_host_permissions` array for Chrome compatibility and in the `optional_permissions` array for Firefox/Safari compatibility.

Another example is that currently only Chrome supports `background.service_worker` in stable releases. To work around this, developers can write their MV3 background scripts in a way that's compatible with both service workers and event pages, then declare both in the manifest like so:

```json { "background": { "scripts": ["background.js"], "service_worker: "background.js" } } ```

Yep, that's it! I settled for generating the manifest.json through a script, since at some point adding something only supported by one browser would cause an error in the other, though I think that may have been remedied now.