Gmail recently added an auto-save feature that throws a copy of a mail-in-progress into the Draft folder, protecting it against browser crashes or inadvertant tabbing away from the Compose screen. The feature didn't seem to work when I first tried it; I dashed off a couple of lines and closed the Firefox tab without sending. Returning to Gmail, the draft was not to be found. Trying again, and writing a longer excerpt, I observed a notice that the draft had been automatically saved. I closed the entire browser, rebooted it, went to Gmail, and found the partial mail in the Drafts folder.

At this point I didn't know whether the successful save was due to closing the browser instead of the tab, the longer excerpt, or perhaps a tie-in between the auto-save feature and my computer's clock.
I proceeded to slowly build an e-mail, a few characters at a time, to see whether the feature would kick in after a certain length had been reached—all the while keeping my eye on the computer clock. The ticking clock did not invoke the auto-save notice as minutes turned over. However, the draft got saved in a burst of typing that brought the character total over about 50 (an approximation; I didn't count exactly).

So, auto-save works nicely, but only if you've typed enough for Gmail to consider it worth saving.