Have you ever seen it open an attachment?

The danger you're being advised about relates to email clients that run scripts embedded into email messages. Thunderbird doesn't (cannot?) do this.

only started using thunderbird today and got an email with a series of photos underneath this line so i thought it was an open attachment and geeks say open attachments can be preloaded with viruses.... mabye i over reacted ...

Really really cautious people do some rather awkward things as a matter of course, such as viewing all messages as plain text or taking Thunderbird (or their computer!) offline before opening any messages. This is a guard against web bugs, tiny pictures which download from a remote server. The fact that you have triggered this download indicates that you have accessed the message and therefore your account is alive and active. This can be valuable knowledge for spammers. Some pictures might be able to trigger the running of a script, but as I said before, Thunderbird doesn't run such scripts.

In all the years I have been using Thunderbird, I haven't found any sign of trouble due to my using the Message Pane. I work through the messages shown in the open mailbox folder (e.g Inbox). It's usually fairly clear which ones are not worth opening. I always wonder about the safety of working the other way, by double-clicking on a message to see it in its own tab or window, then using "forward" or "next" to blindly move to the next one, without any attempt to preview it and judge what it might contain. So I consider the Message Pane (the one you suggested is a preview) is actually safer than using the separate view.

Seasoned users would take issue with it being called a "preview" because it can be used to read the whole message; it isn't just a snippet, and many of us use it almost exclusively to read our messages. But having said that, you will find that some rich-content messages do show a more complete rendering when seen in their own window or tab.

The F8 key (on a PC) should toggle the Message Pane off and on, though some users report (and complain) that it doesn't stay switched off. You can also find this in the old menu under View|Layout|Message Pane.