Yesterday, Mozilla Messaging released Thunderbird 3.0.1, its first security, stability and maintenance release after the initial Thunderbird 3 release.
While it’s always a good idea to quickly update to such a maintenance release for security reasons, this release also contains one important fix for Lightning users, who like to use Lightning in combination with other popular Thunderbird extensions (Enigmail, Quick Folders, Buttons!, etc.). Without this fix, some of our users were affected by bug 398702, which caused the File, Edit and View menus to disappear.
This bug has now been fixed in Thunderbird 3.0.1, which is why we recommend this update to all our users of Lightning 1.0 beta1.