Hi, I'm one of the Firefox for Metro developers. Right now there's no UA difference between the Metro and desktop modes, and I'm not sure if there's a good way for web developers to detect the lack of plugin support when running in Metro mode. I've filed this bug to investigate and fix this problem:

https://bugzilla.mozilla.org/show_bug.cgi?id=938722

You might also be interested in this bug; we'd like to have the Metro browser automatically provide a useful message to users on pages that require plugins:

https://bugzilla.mozilla.org/show_bug.cgi?id=936907

See also this old discussion about possible changes to the UA header on Metro, although we haven't actually implemented any of the proposals there:

https://groups.google.com/d/topic/mozilla.dev.platform/tT-oz7yJ6VM/discussion

In general we don't like broadcasting information in the UA header as part of every request, if we can add a more specific API instead.

Is there any way to modify the user agent string for metro? I am using this for a single purpose on Win 8.1 Pro tablets to display an intranet site for our users. I would modify the site myself, but I don't think the vendor would support us anymore (lol). All I need it to do is use a tablet UA string like a Galaxy Tab or something so this particular intranet site will feed it the tablet version, similar to what you can do with the desktop browser with a user agent switcher add-on.

Edit: Ok, so it's possible to add the "general.useragent.override" preference in about:config (metro-prefs.js), but the change won't stick after you close the browser. How do I make the setting stick so I don't have to add it every time I start the browser?

Edit2: Ok, so changes have to be made to the js file while the browser isn't running, then they'll stick.