You can get the Portuguese version of Firefox 4.0 RC1 from http://www.mozilla.com/en-US/firefox/all-beta.html

I know about that, but that's not what i asked. I have it already installed and just want to change the language. Also, because i'm not sure how to uninstall it on Ubuntu

If you do not have the pt-PT language pack then you can add that pack if you do not want to uninstall the English version.

• http://releases.mozilla.org/pub/mozilla.org/firefox/releases/4.0rc1/linux-i686/xpi/
• http://kb.mozillazine.org/Language_packs

Set the pref "general.useragent.locale" to the name of the preferred language XPI file (e.g. "pt-PT") on the about:config page.
You can open the about:config page via the location bar, like you open a web site.
Use the Filter bar at the top of that page to quickly locate the pref.

Well, none of those tips are working. When i try to install de XPI file i always get the message that is "not compatible with the "b13pre" version.

I uninstalled the app, using the apt-get remove Firefox-4 command at the command line. Then i installed again using the apt-get install Firefox-4 command.

Tried to change the locale to pt-PT as you suggested, and still doesn't change anything.

How can i install the pt-PT package from the mirror, manually, from the start?

XPI won't work, because you have install the language pack in ubuntu, for Spanish this worked for me:

$sudo apt-get install language-pack-es*

$sudo apt-get install language-pack-pt* #for portuguese

Or choose to install it from System -> Administration -> Language Support

Restart Firefox & voila