I had the same problem with Ubuntu 12.04. It seems that the WebMail and Libero/Hotmail/Gmail plugins are incompatible with something that is changed in TB, starting in v15.
My temorary fix is to downgrade Thunderbird to a previus version. TB 14 should be ok, but unfortunately my synaptic package manager propose only to go back to TB 11. Anyway with TB 11 now all work as before. Remember to force the downgraded package version in synaptic to avoid again the update at next run of update manager. This in the hope that the creator of this useful plugin will have time to debug and correct the issiue...
