Sophos UTM Firewall Management Interface via IE or Firefox

Hello,

I cannot get the Sophos UTM Firewall Management Interface to open and log me in.
The problem is I think the certificate prompt, which happens because the connections is over HTTPS, and we of course don't buy certificate for a management console...
This can be either:
Ignored in IE or
Added as exception in Firefox.

My main problem is the entry of credentials, which sorry to say, ain't working at all. I can't get it to work neither in IE or Firefox, trying already different settings. Would I also need a password safe for this to work, because if I use an RDM extension in Firefox, the credentials are not recognized.

Maybe there is some experience on this matter or any kind of help?

Thank you.