Ok, if Gmail was an app in the Google Chrome store only then that might be a problem but this is "Offline" Gmail which I'm sure is using Chrome-specific API's to do it's magic. Google used to have thing called Google Gears (or something like that) that was a framework that would allow for things like Offline Gmail in FF but if I had to guess they either didn't have great adoption or where too limited in what they could so they dropped it and rolled it's features into Chrome.
What I'm not a fan of is "Lowest-denominator web is the best web" arguement. It's not and I'm sorry but unlimited backwords-compatibility is a pipedream, at some point you have to move forward.