Gmail was on an HTML5 roll this year, adding drag-and-drop attachments, image insertion and windows staying open after Gmail closes. Those features were Chrome-only, but have now made their way into Safari 5 on Windows and Mac.
Presumably, any browser that supports these kinds of HTML5 features could get such support in the future — Firefox, Opera and (from what we've seen) Internet Explorer 9 included. Another nice thing we hope makes the grade soon? A reborn Offline mode.
HTML5 features now in Safari too [Official Gmail Blog]