I am guessing that when you say Firefox is "localising" into Portugese, you mean all the menus of Firefox are in Portugese. If so, looks like when you downloaded Firefox, it detected that you are in Portugal and downloaded the Portugese version.

To fix this problem, you need to go to this webpage http://www.mozilla.com/en-US/firefox/all.html and download the Firefox of your preferred langauge.

Some sites automatically redirect you to a site in the local language, based on where your IP locates you.
There is usually a link on such websites choose a language or go to an international version that allows to chose the language.

You can click the link at the bottom of the local Google page to go to the "Google.com in English" site.

Such settings are stored in a cookie on your computer that you need to allow: Tools > Options > Privacy > Cookies: "Exceptions" or Tools > Page Info > Permissions.

• Google.com in English - always '.com': https://addons.mozilla.org/firefox/addon/51000