Hello Quoc.

Although possibly not related to your problem, I have to remind you that the version of Firefox you are using at the moment has been discontinued and is no longer supported. On top of this, it has known unpatched bugs and security problems. I urge you to update to the latest version of Firefox, for maximum security, stability, performance and usability. You can get it for free, as always, at getfirefox.com.

As for your question, you just have to download the desired version from Mozilla's site. If the selected version is not the way you want, simply click "Other systems and languages". Which will lead you to this page: http://www.mozilla.com/firefox/all.html

If the Google (Firefox) or Yahoo home page is in the wrong language then see: Google: http://www.google.com/support/bin/answer.py?answer=873 Yahoo: http://help.yahoo.com/l/us/yahoo/homepage/troubleshooting/troubleshoot-02.html