When Gmail first launched its Inbox mail platform last October, it only worked in Chrome, on Android phones and on iPhones. Google has now added support for Safari and Firefox, and introduced tablet-optimised versions for iPads and Android tablets.
The browser update also means you don't have to use user agent switching to make Inbox work in Firefox or Safari. (Still no option if you're using Internet Explorer.)
Inbox by Gmail: now in more places [Official Gmail Blog]