From MozillaZine Knowledge Base

This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).

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.

External links

Debian bug report #357623 thunderbird: myspell recommends should be depends.

Ubuntu bug report #35212 in mozilla-thunderbird (Ubuntu): "unable to compose messages"