Same problem reported here (it's not related to iframes only, but general the sendMessage API): https://developer.apple.com/forums/thread/659681 The only way around would be to use connect/port API. But it's not exactly easy to refactor existing code... One could maybe build a polyfill that would use connect/ports. Anyway, is this the right place to report bugs? This is 100% a bug.

Hello @arnoudkooi, Did you managed to find a solution for this? I'm trying to solve this for a whole day now. When I send a message with browser.tabs.sendMessage to a tab that I've created it won't receive it at all. No matter if I send it from a different tab or from a background script. It will receive messages send through the browser.runtime.sendMessage though. It feels impossible. I've tried also chrome namespace but with the same results. Is everyone else using browser.tabs.connect API?

oh man, I've used a wrong tag, it should be "Safari Extensions", not system, can some admin change that?