The tablet is brand new so rooted methods are not an option. I seem to remember there being a Firefox add on that had a similar function, which is now dead and and buried. Are there any other tricks out there?
If you can afford to switch from Firefox to an another browser, I suggest you pick any one of the following:
The said browsers feature inter alia universal user agent/ website preference for websites visited and also permits overrides per website. I have tested all of them in Desktop mode.
Note that Opera may require a restart of itself for settings to take effect.