i confirm: disabling lightning (4.0.2) eliminates the problem. but with lightning enabled thunderbird is almost unusable.
if it's so hard to find the problem maybe it would be a good idea to add some kind of profiles into the thunderbird. when user feels some problems, he turns on the profiler and call stack is being sampled. it should help find out which part consumes cpu
i confirm: disabling lightning (4.0.2) eliminates the problem. but with lightning enabled thunderbird is almost unusable.
if it's so hard to find the problem maybe it would be a good idea to add some kind of profiles into the thunderbird. when user feels some problems, he turns on the profiler and call stack is being sampled. it should help find out which part consumes cpu