If it's an extension, it probably still works just fine with Firefox 5.0. However, because of Firefox' poor compatibility design, until Trend Micro 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 the maximum compatible version, which is probably 4.0 in your case 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.