I tested the Compare MoodleDocs add-on with Firefox 15 on Ubuntu 10.04.
I logged in to Moodle Docs and clicked the edit tab (rather than the view source) and then right-clicked the text area and chose a different version of Moodle Docs to compare with. The diff checker opened in a new window, enabling me to easily compare the different versions. Everything worked perfectly :-)
Thank you very much Martin (birdy1976) for this very useful and easy-to-use tool for comparing Moodle Docs versions :-)