How do I change the language of Firefox 4?
You need to download the English version. The version you have will be the Hebrew one.
http://www.mozilla.com/en-US/firefox/all.html
Download the English version. I don't think it's possible to change the language of an already installed browser.
You can install a plugin with the right language. All languages for Windows are found here. Browse around for other platforms.
Also you can find installers there for every language separately.