While writing a message using Firefox suddenly my gmail login details, including password, were pasted into the body of my message at the current cursor position and in the browser's find field (whole password and a bit of email address in the latter). I don't store my passwords in the browser nor in any of it's addons, although I keep it in one of Chrome's extensions. I don't use whole email address to login, only the user name, so it's very strange and worrying.

I must admit I clicked some suspicious link the same day (by checking the root of the domain first), but I quickly closed that page and after that I haven't used that password.

Both Avira and Spybot didn't detect anything.

What was it and what can I do to make sure my browser is safe?