Ironic that ads are what Google uses to pay Mozilla so you(we) can use Firefox.

You assume that Firefox can't exist without Mozilla. Thunderbird got a lot better after divorcing from Mozilla.

A) Firefox is vastly bigger and more complicated than Thunderbird

B) Email protocols aren't a moving target to the degree that the web is

C) Thunderbird has the benefit of being able to freeload off the base platform development that Firefox continues to do, although of course it's a lot of work even to adapt to those changes.