Currently Agoric’s SES relies on a polyfill of the Realms API that relies on the iFrame API, which is not available in ServiceWorkers at this time, which the proposed manifest v3 currently requires all background scripts to be....
All we really need is the FrozenRealms API, since it’s the simplest way to truly isolate foreign code in a secure way, but keeping iFrame support would at least let us continue using the SES Realms polyfill. Our background script must handle user keys which manage cryptocurrency accounts, often the most security sensitive bits of data on the user’s entire computer, so it should be understandable that we care very much about the security available to the background script in particular.
See full letter from Dan Finlay, a lead developer at MetaMask. We’re a chrome extension crypto wallet and account manager with about a million active installs letter 