This is usually caused by a Firefox add-on that is overwriting the browser's User Agent string. This is not recommended behavior for add-ons, but sometimes it is done, usually to force compatibility with a newer version of Firefox. To find which add-on is causing the problem, enter about:config into your address bar and search for Agent. You should see a line where the User Agent string is being overwritten. Once you find the add-on that is causing the problem you should try upgrading the add-on. If that doesn't work, you can either disable the add-on or disable the about:config entry that is altering the User Agent string.
UPDATE 23 August 2009: ProtoShare is fully supported in Firefox 3.5 and Safari 4. We recommend upgrading to one of these browsers for optimal performance.