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I don't know what it is, but GMail seems to work fine on IE, and I presume it works fine on Firefox, and I would assume it almost certainly works fine in Chrome.
Opera presents such weirdness as making clicking on text fields in the New Compose cause focus to go someplace that I have yet to figure out, but definitely isn't the text field. Also, hitting tab doesn't go to the next GChat tab, it goes forward by two. So you can only effectively tab through them if you have an odd number of GChat tabs open. And sometimes when I scroll down, I scroll the whole GMail view (sidebar, inbox, topbar, and all) off my screen; I have to use middle-click to get it back sometimes.