Sizzy started as an extension, but the extensions API is not powerful enough for all the stuff that we’re doing. Please check out what Sizzy can do before you say “this feature”, because there’s plenty of features that differentiate Sizzy from any other browser. We’re really trying to make the most useful browser for developers.

I've seen this in Firefox dev tools. Can't seem to find it now.

They moved it to the console under a gear icon.

Thanks. It's under dev tools > console > the phone logo > responsive

You can't view multiple devices simultaneously in Chrome