The Google Toolbar still works just fine with Firefox 5.0, even though it says it's not compatible. However, because of Firefox' poor compatibility design, until Google releases a new version, you need to install Mozilla's Add-On Compatibility Reporter extension from https://addons.mozilla.org/en-US/firefox/addon/15003 and restart Firefox twice. Each extension specifies which is the maximum compatible version, which is probably 4.0 in your case ('your' as in '90% of users') as Firefox did not give devs any time to verify and re-release their extensions for version 5.0. The Add-On Compatibility Reporter extension basically tells Firefox to ignore the maximum compatible version so the extension can still work on version 5.0, just like it did on 4.0.