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. :(
Tempting, but the set of stuff we would likely need to change might be ... large. Including explicitly breaking some things that are broken in Chrome/Edge and that we'd rather not break, I expect.