Rated 3 out of 5 stars

If not already implemented, here are probably missing options:

-sure it's missing:
X-Priority (during saving)

-if not added yet: saving attachments too,
in emls, inline or simply attached

This review is for a previous version of the add-on (1.2). 

X-Priority isn't possible now with the WebExtension API limitation. If that's change in the future i will add this ;)

Support of attachements are ... "normally" ... done in the newest release 1.3 (this is a first implementation, i've done somes tests, but it's not impossible you can find somes bugs)