The Thunderbird team recently surprised the Lightning team with compatibility issues. Thunderbird 2.6.x broke something concerning the Lightning calendar extension, but apparently only under Linux. The Lightning team performed magnificently, putting out quick fixes within a few days of each issue. The Lightning Project blog has an excellent explanation and a version compatibility matrix. I recovered my calendar capability on Thunderbird with that post’s help.
That’s what I love about open source – pride in their product and rapid response to issues.