You can't do that. Only separate pages or frames can be opened in new tabs. The whole GMail package is one integrated environment. It's not different webpages in frames.
In the bottom of the screen you can find the basic HTML view option. Click on it, and the display will be changed to HTML without java scripts. Here you can open any messages in multiple tabs.
Set keyboard shortcuts to on in settings. Then press <shift>+o to open in new windows. Firefox's popup blocker blocked it by the way, and I had set mail.google.com to allow.
Hi Guest we just wanted to alert you to a major change in the forum. We will no longer be allowing the posting of outgoing links. Please use the attachment feature to attach media to your posts.