Only improvement I would like would be to change the directional options to function like "rotate right from where the image is oriented now" instead of the current "rotate it to the right of it's original orientation", but it's a minor issue. There also seems to be a bug which prevents you from rotating from left to upside down, you have to go from left > right/reset > upside down instead. Pity it doesn't work with the new (>56) firefox.
I installed that version of Menu Editor to see if there was any interference between the two extensions, but the Flip and Rotate menus still appear on right-clicking an image, and both menus appeared in the Menu Editor, so it might be another extension involved. You can create and switch to a fresh Firefox profile and just install Flip or Rotate Image to see if it works there (see http://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles#w_starting-the-profile-manager ) and from there find the culprit extension or setting causing it in your original profile. If you need further help, these reviews are the wrong place to ask, click on my homepage or just visit irc://irc.mozilla.org/firefox in a good IRC client.