An error occurred while creating a message compose window
From MozillaZine Knowledge Base
Some Linux users have gotten a "An error occurred while creating a message compose window. Please try again" error message when trying to compose a message using a third party build of Thunderbird. This was due to a package dependency problem causing it not to install the MySpell dictionary that Thunderbird required. Installing a MySpell dictionary (myspell-en-us etc.) fixes the problem.
Its possible the same type of problem could occur with Windows builds from the Third Party/Unofficial Builds forum.
Debian bug report #357623 thunderbird: myspell recommends should be depends.
Ubuntu bug report #35212 in mozilla-thunderbird (Ubuntu): "unable to compose messages"