Some Gmail users have cleared their mailboxes.

On December 19, in Google Groups, someone posted a message stating that his account was completely cleared. Other users connected to the conversation, who said that the same story happened to them.

Many people noticed that users of Firefox 2.0, and at the time of mailbox cleaning, their browser was open just on gmail.com.

The cause of the problem is not clear. One of the victims claims that immediately after deleting the mail, he received a letter stating: “This is not a mistake. All your mail with contacts have been specifically deleted. This is an attack, not a mistake. Have a good day. =) "Another user pointed out a vulnerability in Firefox 2.0, which was fixed in Firefox 2.0.0.1
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December 22, 4 days after the tragedy, a Google representative left a message in the forum thread:

Thank you all for letting me know about the problem. We are sorry. At the moment we understand the problem and let every user from this thread know about the results.

By the way, according to the official policy of Google, remote mail can not be restored.

Taken from here