I've encountered https://finance.yahoo.com
Well this one is wierdly broken. Firefox on Windows (44.0.2) works fine. Firefox on NetBSD 7 also 44.0.2 gets the mobile site. So this isn't so much a firefox issue as a firefox NetBSD issue that's very specific to this site. Have you made any attempt to talk to them about it? Even more weirdly when I hit that site in Microsoft Edge I get a link promoting Firefox!
The google one seems to think I'm browsing with chrome and doesn't offer a chrome download but apart from that it renders the page fine and has exactly the same content as the windows version of firefox.
My firefox builds are:Build identifier: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:44.0) Gecko/20100101 Firefox/44.0
Build identifier: Mozilla/5.0 (X11; NetBSD amd64; rv:44.0) Gecko/20100101 Firefo
x/44.0 Mike