They're not that hard to code, but both Google and Mozilla keep making changes and then bitching at you through their app stores to change your thing. I recently dropped Chrome support on an add-on because 90% of the users were on Firefox. Since it blocked some ads, I figured Google would continue to whine, and it wasn't worth the hassle.
On the gripping hand, when the platforms in question are the dominant platforms of, err, all computing in this era, then the above described issue tends to be a minor one, in context, for many applications.
With the manifest v3 update, we're changing our freemium structure, and updating our docs. But since FF hasn't figured out all of their manifest v3 stuff, we're not able to update for that 10% of users. It's going to be confusing because our marketing material will reference the new freemium model. But it's no contest as to what we should primarily reference, since there are so few people on FF by comparison to Chrome.