I use Firebird too, with the User Agent Switcher extension installed so with one click I can pretend to be using IE if I hit a website with a stupid test like this.

But when I went to the M&S website just now without spoofing the User Agent, it worked fine, as did running with user agent set to Opera. Looking at the source for the page I didn’t notice any test for user agent.

Perhaps this rant caused them to finally see the light?