[camgunz]

OP spends a whole paragraph describing the effort Mozilla expends to ensure devs are able to target at least Firefox in addition to Chrome:

> We have a Tech Evangelism bugzilla component for outreach to sites who use techniques that don’t translate across browsers. When new sites appear that deliberately block Firefox (whether because the launch team took the time to test with Firefox and determine the user experience wouldn’t be acceptable, or because cross-browser compatibility was an explicit non-goal), Firefox engineers go find the performance cliffs and fix them. Mozilla has a long-history of promoting the benefits of multiple implementations of the web platform; some of the old guard might remember “Works best in all browsers” campaigns and the like. If you squint properly, you can even see this promotion in the manifesto (principles 2, 5, 6, 7, and 9, by my reckoning).

I think it ends up that sites happen to mostly work in most JS engines because most sites don't do a lot of difficult stuff, and if they do they're using libraries that work pretty hard on cross-browser compatibility. I definitely don't think significant effort is spent targeting Chakra, for example. I think most FEs would laugh at the mere mention. Most I know push back against even considering Firefox compatibility.