Review for Thunderbird Conversations by Rigs
Rated 5 out of 5 stars
Excellent except...in-message links that open other pages don't always work. Specifically YouTube links don't open. It says Loading Message but does nothing. Choosing 'View using the classic reader' solves it. A very good extension however.
Upon further investigation it appears there's a conflict with 'ThunderBrowse'. Disabling the latter causes all links to be opened in the default browser, YouTube included.
To create your own collections, you must have a Mozilla Add-ons account.