I do not believe you can stop the rendering itself.
The 3 things I know of that you can use are...
View > Message Body As > Plain text
Not use preview (f8 toggles on/off)
You can also enable Tools > Options > Advanced > Privacy > Block loading of remote images to stop loading remote images (that also kills the ability to execute code from there).
I do not believe you can stop the rendering itself.
The 3 things I know of that you can use are...
View _> Layout _> Message Pane does the same as F8 (#2 above) in Tbird