I'd do it externally in a specialised graphics program such as irfanview.

There are addons for Thunderbird for image manipulation but I think they only re-size. I'd be wary of doing rotation in Thunderbird since it might just mess with the exif data and so would work only in programs that honour exif. If I resize or rotate in Irfanview, I can be confident that the recipient will see it as I intended, and not at the whim of whatever they are using to display the picture.

The picture I am inserting into Thunderbird is in the orientation that I want it but Thunderbird rotates the picture 90 degrees when it is inserted. I tried saving the picture in another file with the picture rotated 90 degrees and Thunderbird still inserted the picture exactly the same.