From MozillaZine Knowledge Base

Article first created

This article is a copy of Changing_Firefox_windows_back_to_classic_mode with the extension, about:config, and external links removed, with very little changes on top. I installed TB2.0 today and noticed it has the new-style menu's, so I tried the FF code and it worked. I do not remember if TB1.5 had them, I do not know of any applicable external links, or if there is an extension.

Unlike Firefox, the nature of TB's about:config window means userChrome.css code works on it and userCOntent.css code is necessary. More background info on this change in Thunderbird is needed.