Unfortunately the web keeps changing. Firefox is trying hard to remain relevant and needs to be flexible with their code-base. So Thunderbird does not live in a stable world.
In TFA it says that Thunderbird is built on top of Firefox, which makes it difficult for them to keep up and causes a lot of churn and bugs. I imagine that’s what the parent poster is referring to.
Yes, and today, to render mails, you need a web engine. Unless you only want to deal with text email but that comes from with its fair share of tradeoffs, including the occasional unreadable email in text… which you'll open in a web browser.
And as a mail client you'd better follow the security fixes of the web engine you are using.
I guess not everyone is willing to use an old web engine derived from an old version of Word and maintain this thing for the eternity.
