Rated 3 out of 5 stars

Cameron's website is gone si I am guessing that this dictionary is abandoned. If I could figure out how to take the extension over, I would because the fix to make it install on Thunderbird 45 is trivial.
Here is what I did:-
1. Download the extension to local hard disk.
2. Using an archive manager, extract the files to its own directory.
3. Edit install.rdf and adjust the following lines:
- em:version 2.2 /em:version
+ em:version 2.2.1 /em:version
!-- Firefox --
- em:maxVersion 33.* /em:maxVersion
+ em:maxVersion 50.* /em:maxVersion
!-- Thunderbird --
- em:maxVersion 33.* /em:maxVersion
+ em:maxVersion 45.* /em:maxVersion
4. Save file
5. Edit install.js and adjust the following line:
- var err = initInstall("English (Australian) Dictionary", "en-AU@dictionaries.addons.mozilla.org", "2.2");
+ var err = initInstall("English (Australian) Dictionary", "en-AU@dictionaries.addons.mozilla.org", "2.2.1");
6. Save file
7. Select all files and "dictionaries" folder and create a new zip file.
Make sure it uses "deflate" as the compression type.
I called the zip file english_australian_dictionary-2.2.1-tb+sm+fx.xpi which is just a rename of the saved zip file changing .zip to .xpi
8. Go into thunderbird add-ons and using the little gear icon, select "Install Add-on from File", choose the new xpi file and it will install.
By the way, it also seems to work in Firefox 50.x

This review is for a previous version of the add-on (2.2).