Some websites load as mobile versions (firefox 3.6.13 Maverick)

I think there is probably some bug with firefox in 10.10 (Maverick).
Problem has shown when upgrading from Lucid. Never happened before.
I've seen several questions about this and even found some old bugs, none of them apply to my problem.

This is my user agent (unchanged default one)
Mozilla/5.0 (X11; U; Linux i686; es-ES; rv:1.9.2.13) Gecko/20101206 Ubuntu/10.10 (maverick) Firefox/3.6.13

Sample problem site (Spanish newspaper):
www.elcorreo.com

Workaround.
Change user agent removing '(maverick)', for example with User Agent Switcher extension.
Tried also the Lucid user agent on Maverick and works OK, as I knew from my previous installation.
Windows user agent works fine too.

Can anybody review this? Is any 'm' in the user agent, like the one in 'maverick' interpreted as mobile?