Was a solution to this discovered. I started experiencing it last night in Firefox on MacOS. Works fine in Chrome. I was able to get the error to go away and work normally by disabling LastPass. Ideally I would like to have use of that add-on back as opposed to switching browsers. Is there anything uncovered as to why Last Pass would be the issue? If it helps here are my Firefox and Mac versions...
--Firefox 68.0.1 (64-bit)
--macOS Mojave version 10.14.6
This started around 4pm July 25th. I reinstalled Firefox this morning to confirm cache cleared and latest version installed. Works with all other plug-ins active except LastPass gives the "error displaying content preview".
I called support and we figured it out. It was an extension that was added under Firefox. Go to preferences and click extensions. I only had two .. but I can't remember the one that was giving me problems. So sorry. But I did keep my adobe and that's it. Once I cleared my extensions under Firefox, the error message was gone. Hope it works for you!