Is it possible to customise the web browsers used for previewing HTML emails. I would like to add Firefox, and if I explicitly choose 'Safari' the message previews in textedit, which doesn't seem right.
We will likely add that feature. About your problem, try the following. Create a text file. From the finder change the extension to '.html'. Then double-click on it. What application is used? If it is TextEdit then it is because the wrong viewer is set for that extension. Just select the file, press option+I and select 'Safari' as the default viewer.