Quick post to add some Google juice to a problem. If you upgrade to Thunderbird 24 and it no longer underlines your misspelt text in red, go to Tools -> Preferences -> Advanced -> Config Editor and set mail.compose.max_recycled_windows to 0.

References

This reference list has been generated, in many cases, through an automated process and may therefore contain errors or incomplete metadata. It is provided purely as assistance for finding the items mentioned within this post.

  1. Mozilla Bugzilla fix for Thunderbird spellcheck bug <https://bugzilla.mozilla.org/show_bug.cgi?id=880595#c27>