yeah - latest versions of firefox (52 onwards) now winges at you if the website is http and not https.
There is nothing you can do about it, might be a way to switch the message of - haven't looked yet.
But that's all it is.

It's up there with the internet explorer message that always used to ask you if you wanted to debug the website. Since you couldn't - why ask ?
This is similiar in that there is nothing you can do except ignore the message.