For more backstory, here's the Firefox bug report with the fix for Google Inbox being "too slow" due to JS array slice being faster in Chrome than Firefox due to a V8 bug:

https://bugzilla.mozilla.org/show_bug.cgi?id=1087963

Here's the Firefox bug report about Google Docs being 3-9% faster, in a particular performance benchmark, when Firefox spoofs the Chrome UA:

https://bugzilla.mozilla.org/show_bug.cgi?id=1307024