You have to download and install both versions first, then you can use the following add-on to switch between those two versions of Firefox:

https://addons.mozilla.org/en-US/firefox/addon/1333/

I hope that's what you are looking for.

It's perfect, thanks :)

This thread got bumped up recently (possibly as a result of the major update). Was there a particular set of locales that were affected by the major update? -- were you using language packs previously or actually using the downloaded Firefox built with the given locale?

I am in a similar situation: I have Firefox in Spanish - I want it in English - and I DON'T want the Spanish version - so I don't need to be able to switch back & forth - cause i ONLY want it in my computer (a Mac) in English - so is there a way to SWITCH it to English without uninstalling the Spanish version completely (and consequently, having to freshly install Firefox, in English, all over again)? Thanks

You need an English language version and there is no en-US language pack in Firefox 3.6. If you do not want the download the en-US version then you can install the en-GB language and switch to that version and remove the Spanish es-ES language pack from the chrome folder in the Firefox program folder.

You can find all language pack here:

http://releases.mozilla.org/pub/mozilla.org/firefox/releases/3.6.11/win32/xpi/.

You can install language pack just like extensions.
You can set the pref "general.useragent.locale" to the preferred language ("en-GB") 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.

See also http://kb.mozillazine.org/Language_packs

If you need the en-US dictionary then look here:

• https://addons.mozilla.org/firefox/language-tools/

I installed the languagething but it doesn't change ALL the system to english. I downloaded here in Korea, and there wasn't an indication for an English only version, so now my tabs and such are in Korean. i'd like to change it so NO Korean comes on unless I (or a friend using my system) wants and needs it. Do I have to start all over and download english language version and if so, where do i find it while here in Korea?

You can find all Firefox versions here: http://www.mozilla.com/firefox/all.html

1. get the language pack you want:

• -> go to: http://releases.mozilla.org/pub/mozilla.org/firefox/releases/
• -> find your release (example: 4.0b12)
• -> find your system (example: win32)
• -> go to the folder xpi (example: en-US)
• -> click on the language pack you want.

2. change the language

• -> go to: about:config
• -> I'll be careful, I promise!
• -> search for: general.useragent.locale
• -> modify it to you're language pack (example: en-US)

3 restart Firefox

If you installed the English-US version, like me and accidentally changed it when I was trying to change the size of the chat box, here's what worked.

In the bottom right-hand corner is your location, click on that and look for whatever option ends in "en-US." I accidentally switched to Russian, and was still able to see the "en-US" option. After clicking that, I was able to click on what I figured was the "OK" button and got English back WITHOUT re-installing or adding any language packs.

The explanation saved me. It wasn't clear that by selecting the language pack I would be reinstalling Firefox, but it seems that it only installed the differences between the existing version and the new one I downloaded. In the end, it works, and on the about:config page I can see the en-US instead of en-GB. Thanks again. Ben

''bold text''

I still cant do anything with Mozilla cause its all in spanish and gives no way to change it even with ad ons