I've got Firefox 3.6.12, but on sites like YouTube I've got a message:
Your browser is not supported here
Please, help me!
Try resetting your user agent string (identifies the version of Firefox being used). Some add-ons can change the user agent string and that can lead to error messages about unsupported browsers. For details on how to do that see the Websites say that Firefox is outdated or incompatible even though it's the latest version article.