Uh.... user agent....
I thought I rember seeing don't mess with the user agent written somewhere...
I've never set that in my profiles... should I be resetting it to something new for each profile?
No it's not mobile.
Try changing it in profile settings.
Here are couple user-agent strings:
Mozilla/5.0 (Windows NT 5.1; rv:52.0) Gecko/20100101 Firefox/52.0
Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:53.0) Gecko/20100101 Firefox/53.0
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Firefox/45.0
Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:52.0) Gecko/20100101 Firefox/52.0
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:40.0) Gecko/20100101 Firefox/40.1
Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Firefox/52.0
Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:53.0) Gecko/20100101 Firefox/53.0
You can find more in google search.