Thunderbird:Help Documentation:Style Guidelines
The guidelines for how Help documents should be written (for this wiki -- there's a slightly different set of guidelines that applies once these documents have been added to the mozilla.org source tree) fall into two main areas: language and coding style. Because the coding style issues are more applicable to when the documents are actuallly in the mozilla.org tree for building, most of the issues covered will be those of language.
Language
Refer to the application as "Mozilla Thunderbird" once at the beginning of each document and in the article title (if you'd want to refer to the application by name), but after that refer to it as "Thunderbird".
Coding
The big rule: don't use wiki markup! We won't be able to use wiki markup when the docs get added to the source code, and most HTML markup is still valid in this wiki.