Why does Trisquel have thunderbird in the repositories, as opposed to icedove?

My understanding was that the reason firefox was rebranded to Abroswer was that the links to non-free extensions were considered an implicit endorsement of non-free software. I do trust that Trisquel would use icedove or another rebrand if the fsf was not okay with thunderbird, but could someone clear up my confusion on what makes something acceptable to be included in a fully-free distribution? Sorry if this is a stupid question. I actually use thunderbird btw.