As it currently stand you would need to either override a boatload of css rules or you could create a duplicate site that is set to "Fixed Width" in the advanced tab of the template manager. Then create links to each version of the site from one another.
This item may be a good post in the Requests
forum. The Dev Team uses that forum for many of it's future development plans.
after looking at it again you can do this be creating links that have queries like this
?layout-mode=1200fixed
and
?reset-settings=reset
you can use this in the menu using the item type = "External Link" and use these as the links.
for the "Desktop View" link user: ?layout-mode=1200fixed
and
for the "Responsive View" link user: ?reset-settings=reset
I have tested this using Kirigami and it works fine and it is a lot less work that my erroneous solution from m first reply in this thread.
I am not seeing an issue with firefox. You should note that once the desktop link is selected your browser will stay in that view until the view is reset by selecting the mobile link.
Yes I understand that you must manually select 'Mobile' after viewing 'Desktop' but I humbly ask that you try again please.
Try this:
When you first go to the website it should default to Responsive.
Now try reading one of the news stories. See how the 'Article' is not responsive?
Then go back to the home page and you should notice that the layout is no longer Responsive.
See what I mean? I've observed this in Firefox, Chrome, and IE