Warning about non-encrypted information when signing into Gmail

Something strange happened recently when I signed in into Gmail. When I had entered the username and password and pressed Enter to sign in, Firefox gave me a warning saying that I was about to enter an encryptated site including some none-encrypted information, and I had to confirm it (by pressing OK). I can’t say exactly what it said, because my Firefox is in Swedish, but I hope you get the idea.

I can’t recall this happening with Gmail before, and I haven’t changed any of the settings or so. It worked normal earlier today when I signed into it (though that was with another account, but that shouldn’t matter).

Does anyone have any idea about what may be behind this? And - most importantly - is it anything to worry about?