Everyone who uses firefox is getting this on most of the websites they login to. The issue is that passwords are sent un-encrypted. The internet is full of posts about ways to get rid of the message, virtually no web sites use SSL except for ones doing financial transactions. Google Chrome does the same
Many people use free wi-fi provided at public places, and it is a potential risk of interception in that case to everyone.
I would make sure that you never use the same password for a site like this as you do for other sites, because, hackers with the desire and resources could intercept login information. The CIA, or other state sponsored hackers can get to almost anything.
I'm not really worried if they get my password and login since they are unique. I use software called Roboform to generate random passwords of whatever length I need, and they can include most characters. This means I can have hundreds of passwords for every login. They can be encrypted and stored on a thumbdrive (s) where they are password protected as well. In a setup like that, you only have to remember one complex password that cannot be guessed.
Since I use Firefox, and it was annoying, I disabled the message.