With it set to FALSE Firefox will still warn you that you are connecting to a dangerous or weak server (UAL) and ask if you wish to continue at your own risk. With it set to TRUE, it forbids you from connecting to such server - it makes the decision for you.

The danger of having it set to FALSE is that you could potentially try connecting to a phishing server unintentionally (a server pretending to be your bank and try to steal you login credentials), and even after being warned by Firefox, agree to continue putting your identity and computer at risk.

If you choose to have it set to FALSE just be sure that you never ignore the server security warning and only agree to continue if you are certain that you are connecting to your intended server (connecting via CCS link or such.)

I use Firefox only for company business (CCS & Skynet); Apple Safari with high security restrictions for all Banking; and Chrome for general goofing around the web - that way even if some malicious tracker sneaks by Chrome, it will remain isolated from my work or banking.