Why? Remember that at this point, Google Docs is "sticky". Its audience is to some degree captive, and won't switch just because the document lags a bit on Firefox. What would they switch to? Office365, for which they have to pay? It wouldn't even solve their problem, as the document in question is a link they got from a friend, already in Google Docs. It's just easier to switch to Chrome.

Most of SaaS applications operate in this regime - they own user data, so they become "sticky" and non-substitutable very quickly. On the other hand, standardization of browser features makes browsers not very "sticky". This way, when a SaaS - any SaaS, not just Google's - works much better on Chrome than on Firefox, this drives adoption of Chrome by that much.