Rated 5 out of 5 stars

Without the custom typeface and font setting in this extension thunderbird would be unusable for me. Without this extension, the composed message would use times new roman when replying to certain outlook emails. This extension is a must have. Other than the typeface setting I also use the custom quote headers.

I'd love to have an option to make the email addresses in the quote headers *not hyperlinked*, or force them to use a black/white colorscheme (hardcoded inline css?) instead of blue hyperlinks.
I am aware of your other extension "Change Link Appearance", but as far as I am aware, it only works locally when viewing emails on TB. I'd be keen for a solution that can work on other clients – by either removing the mailto hyperlinks or forcing an incline css style to make them black or white.

When I get outlook replies from certain senders, their quote header has the behavior i'm after. I think a version of outlook offers this behavior. Because it's not all senders. Would be great to have this here.

That's a great review of you.

Regarding the mailto hyperlink, in the past I already tried to find a solution to remove it but I could not find any, because:

(a) The hyperlinks are automatically added by Thunderbird right after clicking the Send button without this addon being able to intervene.
(b) My observation about messages from Outlook users (as you as well have pointed out) are not consistent. This means even if it were possible to remove the hyperlinks in Thunderbird there would still be a chance for them to be added again by Outlook.

Of course if somehow in the future I get some idea, I'll consider to implement it.