Comment 19 for bug 306610

This is a deeper problem in Thunderbird.

The only thing that really solved it for me, was the QuoteAndComposeManager (former RealBorders) addon:
http://nic-nac-project.org/~kaosmos/realborders-en.html

In the addon settings, under the tab 'Fonts', select the option 'Stabilize predefined font'.

Good luck!
Dotan