Several users (around 60 according to Google) experienced something that could damage the trust some users have in Gmail. The affected users report that all email and contacts in their Gmail accounts were permanently deleted by no action of their own. Michael Arrington wrote about the problem earlier today and received this response from a Google representative.
We saw your post today about Gmail and wanted to let you know what was going on.
Regretfully, a small number of our users — around 60 — lost some or all of their email received prior to December 18th. Once we found out about this issue, we worked day and night to confirm that only a few accounts were affected and to do whatever we could to restore as much of the users’ accounts as we could. We’ve also reached out to the people who were affected to apologize and to work with them to restore the email from any personal backup they might have.
We know how important Gmail is to our users – we use it ourselves for our corporate email. We have extensive safeguards in place to protect email stored with Gmail and we are confident that this is a small and isolated incident.
Google is handling the situation well and doing some damage control by reaching out to affected users. It's true that Google probably isn't to blame for what happened -- Firefox had a flaw in version 22.214.171.124 that might be the root of the problem as Arrington reports. If this is the culprit, users of version 126.96.36.199 should be safe from the attack.