The most noteworthy change is syntax highlighting. It is based on the codemirror library and makes editing script easier.
Some long standing paper cuts have also been fixed. For example an indicator in the tab's caption shows if the script was changed or not. Furthermore you can use Control+S to save the script. A double clicking a script now opens the editor window...
On the back end the add-on is now ready for Thunderbird's upcoming App Menu.
And as always some bugfixes. Closing tabs in offline mode could fail which ended up with unclosable tabs. This is hopefully fixed with this release.
It is currently pending in mozilla's review queue, so you have to be a bit patient. It usually takes two weeks until the add-on is reviewed.