[JKT]

The problem is actually Thunderbird - putting it very simplistically, it isn't a fully 'native' Mac application and as such doesn't support many of the features that the OS offers. One of these is Services. If you copy and paste a page from OmniWeb to e.g. Mail or TextEdit, the formatting will be preserved fine (you can also use the Mail Contents of This Page option from the file menu to achieve the same effect if you are using Mail).

The reason why Thunderbird works with Firefox is because they are both developed by the Mozilla org and both share the same back-end code for rendering.