The toolbar is listed as compatible with up to Firefox 3.6.* A couple of options: # Check with the site to see if they plan to release an update. # Install the Add-on Compatibility Reporter extension ([https://addons.mozilla.org/firefox/addon/add-on-compatibility-reporter]), this extension will allow you to install extensions that are not listed as compatible to see if they work in Firefox 4. No guarantees that the toolbar will work, but it gives you the opportunity to test it.