Some website detect I'm using PC despite user-agent spoofing.
(like developer tools responsive design mode in Chrome/firefox.)
They are checking viewer's platform by another method and here's how to emulate mobile device in these case.
NOTE:Please try this at your own risk.

They are checking your platform using javascript and that's not just user-agent.

If you look up "navigator.platform" in javascript, it will return OS name something like "Win32".
This feature is old and no longer recommended so not many websites using these days, but some still.
So when you face this problem, you need to spoofing "navigator.platform".

You can change "navigator.platform" in Firefox!

OK, input "about:config" in address bar as usual.
(Yes, this is why I always use Firefox...)
Search "general.platform.override", select "string" and press plus icon.
Input whatever you want as "navigator.platform" string and press check mark. That's all.
You can delete setting by pressing trash can icon when you don't need anymore.

You can emulate iPhone/Android by "general.platform.override" string.

If you want emulate iPhone, input "iPhone".
If you want emulate Android , input "Linux".