Forum Moderators: buckworks
Have you checked your checkout process from a different machine? Some users will motor right through that warning, but most won't. Have you made any recent changes that might be suspect?
I should whine here more often. I had a slew of orders this afternoon and few abandonments. ;)
Seriously, do make the effort to get rid of it. Even though it's your thank you page, for a web-savvy customer such a pop-up says "here's someone who doesn't care about his sight" and to an not-tech-savvy customer it might say "hmmm that's a bit worrying, could my data have been compromised".
The quick way to find out what causing the "secure and non-secure" warning in Firefox is to right-click on your thank-you page, select View Info, and then choose the Media tag. It'll show you all your images and you'll probably find the offending item here (it'll start "http://").
If not, then it takes a bit more effort to track down. You may have a script on the page that is sourced from http. More difficult to track down is a script that loads another script, or image, through http. Sometimes scripts create IFrames which again use http. The DOM viewer in Firebug is your friend here.
It sounds like a lot of effort, but you'll probably work it out pretty quickly, and the fix will be trivial. You're obviously sensitive to cart abandonment (and you should be) - in my experience it's the little things that add up.
Obviously that isnt causing your abandons as they only see that page if they complete, but if you are like me you like every detail perfected