Thunderbird uses the exact same application framework Firefox uses, and
therefore the exact same addons facility that Firefox uses. Branded Mozilla
products use the Mozilla addons site, which is known to provide non-free
software. I believe this was one of the issues that prevents vanilla Firefox
from being included in Trisquel.
Trisquel does contain Thunderbird, but it uses the GNUZilla addons site
instead of Mozilla's. Since Mozilla's fairly restrictive trademark policy
prohibits using the official names for modified products, this Thunderbird
might not even be able to be called Thunderbird.
The other problem with Firefox was also that it helps provide non-free
plugins such as Flash. I don't believe Thunderbird ever did this (since
you're not even supposed to be able to use Flash in email) but I could be
wrong.
