Also had similar problem of Kee not being able to connect to KeePass and I just resolved it. So Iam sharing how I fixed it.
Waterfox 56.0.3 (64-bit)
Windows 10 (64-bit)
In my case it was another addon issue.
I was running discontinued RequestPolicy 0.5.28.1-signed.1-signed (https://addons.mozilla.org/en-US/firefox/addon/requestpolicy/). I had no idea that this version was discontinued (installed it some time ago).
And even though I read all the troubleshooting tips which mention to whitelist things in other addons, which I did in every possible addon I have.
Whitelisting it in the outdated RequestPolicy wouldnt help at all .
Installing “RequestPolicy Continued” (https://addons.mozilla.org/en-US/firefox/addon/requestpolicy-continued/) would simply resolve the issue.
Think the new RequestPolicy did read the whitelisted items from the old version, so whitelisting might also be needed.
(Whitelist: Allow 127.0.0.1:12546)
Anyways I first had to find out if its actually addon causing this, and if yes then which one.
Good way (IMO) seems to be simply disabling all addons and trying Kee.
If it works, enable some of your disabled addons and check again. If it works, enable more and check again, until it stops working, then you have much narrower choice of problematic addons.
If it doesnt work with addons disabled, its most definitely not an addon causing it.
I know those are pretty basic troubleshooting steps, but maybe it would be nice to mention such way in troubleshooting info.
I can see you guys here seem to all have Firefox 57.x so you might have another issue and this might be completely useless.
But thought I throw this out there, especially because RequestPolicy is mentioned in troubleshooting along with what to do, yet I still stumbled into such situation, which might make you first assume that RequestPolicy is not at fault after whitelisting. ( surprise, surprise xD )
Good luck ya all