All text in Firefox itself, toolbars, menus, options, etc., will be in whatever language of Firefox you downloaded and installed. Only one language is included in each installer to keep the installer file size down. The simplest route to permanently change your Firefox application language is to download and install a different one from here: http://www.mozilla.org/firefox/all.html Note that you will keep all of your Firefox data when installing a different language. Also note that the latest version of Firefox is newer than what you're currently using so when you download and install the English version from above you'll also get the update at the same time. You may also wish to install a [https://addons.mozilla.org/firefox/language-tools/ Spanish language dictionary] if you wish to have both English and Spanish spell checking available.