There is a dangerous phishing attack being targeted (mostly) at Google Mail. It’s clever enough that it has fooled even sophisticated technical users. What happens is that you get an email from someone you know that includes a clickable image. When you click on the image you are asked to login to Google again. As soon as you do, you’re compromised.
That sounds like a vanilla phishing attack but what is different is that the location bar appears to point to Google so an unwary user will feel safe clicking. If you carefully examine the contents of the location bar you will see that the protocol is slightly wrong. The post linked above has all the details so you should definitely check it out.
The post discusses this in terms of the Chrome browser but from the comments it appears that—at least—FireFox is vulnerable as well. Google is aware of the problem and is working on a fix but if other browsers are vulnerable you will need to be careful regardless of what Google does. Enabling two-factor authentication may or may not help but is always a good idea.
If, like many people, your Google account is tied many other services, having it compromised can be disastrous. Again, you should definitely follow the link and read the post so you know what to look for.