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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.

Cette critique concerne une version précédente du module (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.