[zenlibs]

The definitive answer can be gleaned by setting the user agent to Chrome (or anything besides Firefox). Have you tried that?

[ktpsns]

Changing the User Agent might help to fool a HTTP server, but not the client side scripting, where every browser offers a different API. In web applications such as GMail, this is the relevant piece.

And I confirm OPs assumptions. I also feel this is a tricky way of Google to make Firefox users feel their browser of being "slow". It is basically power abuse. As Microsoft did it in the 90s.

[beckler]

This might not produce the results you expect, because they might be doing feature detection.

[Throwaway_8545]

Google would nearly certainly break the site in such a way that they have plausible deniability.

[craftyguy]

Browser fingerprinting these days goes FAR beyond trivial user agent values.