Rated 4 out of 5 stars

All seems to stem from a beta version (27.0) of Thunderbird. I reverted back to 24.2 and everything is working again, including Lightning 2.6.4. This is still a top-notch add-on but needs brought in line with newer versions of TB and Lightning.

This review is for a previous version of the add-on (0.25).  This user has a previous review of this add-on.