Well, its Firefox time and i found an interesting extenstion called Ajax Yahoo mail Extenstion, Designed by Vivek Jishtu. By Using Ajax Technology, it lets you to preview your message with single click without leaving your inbox.
But Unforunately the new yahoo comet users dont need it, and useful only for classic yahoo mail users. If you feel that Yahoo mail beta is not fit for you, then you can install this Firefox Extenstion from Mozilla Site and make your mailing experience easy…
You can Even Download attachements with single click and send back message using Instant Reply…

This Extenstion works well with Firefox 0.9 - 1.6 all