"Thunderbird in theory IS free software"
Even if you ignore the add-on problem it is not because all four freedoms are not available. You get 3 or maybe 3.5 depending on how you count. Rebranded versions allow you to escape Mozilla's crazy licensing and restore all 4. So "Mozilla Thunderbird" proper cannot be said to be free software, no, since all 4 freedoms aren't available with "Mozilla Thunderbird." Hope that helps. :)
