A bit hazy on the subject, but possibly this all revolves around the subject of
'geotargeting', 'local search' etc. Google can target specific ads to you based on location (although far from perfect I gather for a variety of reasons) and in reverse will provide search results based on a number of parameters; IP addresses, server locations, domain name addresses etc. (There are even websites displaying dual Google results for the same query from different countries).
Maybe your problem originates from this scenario, or maybe some sort of redirection to Google Spain, courtesy of your Spanish ISP's (?). Either way, it's easily resolved;
Keef's previous bookmark suggestion being the quickest.
Are you referring to use of the Google Toolbar as an Add-On to Firefox, or the Firefox Search Bar (in which you can do Google searches, amongst others)? If the former, there is always the option to ditch it and make use of the latter and set up specific individual Google country based search engines, be it UK, Spain, or wherever. See the last paragraph of post #5,
HERE