I created an WebExtension to force Facebook into the old layout - OldLayout.com
It's very simple, but it doesn't seem to work in Opera. So I tested other extensions that change the user-agent header in requests, and none of them seem to work either! I'm using https://manytools.org/http-html-text/http-request-headers/ to look at the actual request headers being sent, and the user-agent header is never being modified, by any extension.
My code is extremely simple, and uses chrome.webRequest.onBeforeSendHeaders.addListener() to intercept headers and change user-agent. Full source is here: https://github.com/matt-kruse/old-layout/tree/master/extension
I can't find any documentation that says webRequest is broken in Opera. But, I am also not a regular Opera user so I don't know if I'm missing a setting or something that is required for these to run? Any tips at all?