But I couldn't disagree more with number 3. Thunderbird is always going to be a niche product with a much smaller potential user base than Firefox.
More or less every corporation and every individual user out there is a potential consumer of Firefox--every single one who uses the web.
Thunderbird? Not so much. Many consumers prefer web-based e-mail clients or e-mail clients tied directly to the e-mail platform they are using (Gmail, Outlook.com, etc.). And Thunderbird is never going to replace Outlook for corporate mail. Thunderbird is only useful when you want a generic feature set, and it mostly competes with other niche mail clients.
Basically, every dollar invested in Thunderbird is a dollar that could be better spent on Firefox (or MDN). If anything, they should spin off development of Thunderbird into a separate company with its own independent funding structure.
