You can see this in the Web Console.

• "3-bar" menu button or Tools -> Web Developer
• https://developer.mozilla.org/en-US/Tools/Web_Console

You can hide other sections like CSS to make it easier to notice these warnings.

While this is better than nothing, it can only be a workaround until something better appears in the Firefox GUI. Also, when I opened the Web Console on several of the tabs I have open that display the shield symbol, the log is initially empty and requires me to load the page again, but Firefox should already have the information what it has blocked. I do not consider myself a web developer, and find this console not very easy to use. (regarding filtering, even only Warnings are more than just the ones related to Content Blocking. I found that a text filter "blocking" is needed to narrow it down.)

I think everyone concerned about privacy should have easy access to the list of sites that they will permit by clicking on the "Disable blocking for this site" button. So my hope would be that "Trackers" and "Tracking cookies" would be a link resulting in a GUI element that shows this list, maybe parts of the "View Page Info" dialog.

firefox 65 which is going to be released later today will contain more information about which tracking elements are blocked in the control center: https://mozilla.invisionapp.com/share/RHORBWDNX4W#/screens