Thanks for your interest in the Google Toolbar.
It's currently available for Internet Explorer for Windows.

Googlebar: We encourage Firefox, Mozilla, and Netscape® 7.x users to install the open-source Googlebar, developed and supported by Google fans. Learn more.

Note that Firefox also has a built-in search box in the top right corner that uses Google as the default search engine.

To learn about the Google Toolbar for Internet Explorer, click here.