Rated 4 out of 5 stars

This is a time saver, and actually works with later versions of thunderbird via the checkCompatibility add on which disables the compatibility checks.
The one issue I noticed as more move to gmail is it does not identify correctly the quoting gmail uses in html emails.

This review is for a previous version of the add-on (0.9). 

Thanks for the nice review.

As for HTML quotations: Thunderbird does not recognize them as such, therefore QuoteCollapse cannot act on them, unfortunately - that is, I have found no way around so far.