First, this is an excellent job, with a very well designed and ergonomic UI with too many small nice things to note. The old Remove Duplicate Messages (doesn't work at all) and Advanced Remove Duplicates (found a lot of misses) can be replaced. I found hundreds of old dupes in the Engadget rss feed (9500 items) which were not found otherwise. Tested in Tb18.104.22.168pre.
Some nits: 1. #Lines column is not sizeable, all others are fine. 2. If a folder is not selected, yet right clicked and Remove Duplicates selected, and the summary file is not built, then nothing happens. Should rebuild summary in this case. Yes, this is an extremely fine point.. (Summaries gone seems to be due to sharing a Tb profile between win and linux file system.) 3. Selecting Message ID column and window/column positions/sizes are not remembered across dialog closes. 5. #Lines is not printed (nice double check even if it's not a selected dupe detection option).
Suggestion: 1. It would be nice if in the Duplicate Messages Deletion dialog, selecting of a message there would have the effect of selecting it in Tb. 2. Move the Delete permanently checkbox above the columns; this way the box size can be styled for max size just with css (and not js).
It would be great to see this officially reviewed by AMO and released.
This review is for a previous version of the add-on (0.1.3).