Add Thunderbird

[ExE-Boss]

Fixes #2008

TODO:

• Account for bug 1455471
• Change manifest.json keys implemented in Toolkit to null
• Change WebExtension APIs implemented in Toolkit to null

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See also bug 1503501

[ExE-Boss]

review?(@Elchi3, @wbamberg): Thunderbird 60 has now been released with support for WebExtension Themes.

[Elchi3]

I'm sorry for the late reply here. I will try to come to decision making earlier in the future.

I have brought this up with the MDN staff team now and we've decided we won't support Thunderbird in this project. Adding more browsers doesn't come with zero costs. The Thunderbird community is small and it's unlikely someone will handle reviews and updates to the data for us going forward.

Further, the core mission of MDN Web Docs is To provide developers with the information they need
to easily build projects on the web platform. We are already somewhat unsure about our support for WebExtensions docs as these are only in theory cross-browser, not web platform tech, and standardization seems stuck. Thunderbird is even further off of our main mission. We (the MDN community and the paid staffs) do not have endless resources, so we have to make some cuts somewhere. Documenting the web platform is already a giant task and MDN has trouble modernizing and keeping up with the ever evolving web platform.

For questions or concerns on this, feel free to reach out to me, or open a new thread on https://discourse.mozilla.org/c/mdn.