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.