Firefox: "this Connection Is Untrusted" Message

I am using Win 7 64 bit Home Premium, with Firefox 23.

I have been using BD Internet Security 2013 for only a few months. I had a bad experience the other day, and picked up a number of pieces of malware. First of all, Delta Search came up instead of Google, and with some difficulty, I managed to get rid of that.

Then, the google url www.google.co.uk was being redirected to https://www.google.co.uk, and I was getting the Firefox message "This Connection is Untrusted". I was also getting this message from myEbay, and other https sites which had previously been ok. Other pages seemed OK, except, some pages, for example https pages from Mozilla, were rendered

as raw html, without the CSS.

I looked for a couple of nights for a solution. Scanned with BD, Malwarebytes and hitmanpro, but still the same problem. Reinstalled Firefox; no difference.

I saw some forum posts which suggested disabling "Scan SSL" in BD, which seems like lowering my security level, but in the end, out of desperation, I did this, and it worked.

What happened to my PC ? What was the cause ?

Is it OK to run without "Scan SSL" ?

Have I reduced my level of security ?

Are there any other solutions ?

I bought BitDefender after reading a couple of reviews, but now, I am a little disillusioned.

Thanks for any advice,