Today we discovered that Closure breaks if we fix https://bugzilla.mozilla.org/show_bug.cgi?id=968056 … without using an IE/Edge/WebKit UA string: <https://github.com/google/closure-library/issues/883 …>. And other Google sites are broken with Gecko's current behavior. So Google is forcing all UA strings to look like IE/Edge/WebKit. :(
I mean, yes, we can just switch to a WebKit UA string wholesale. That does require introducing a bunch of bugs, etc.