Clear the cache and the cookies from sites that cause problems (I know that you've already tried that).

"Clear the Cache":

• Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"

"Remove Cookies" from sites causing problems:

• Tools > Options > Privacy > Cookies: "Show Cookies"

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Thank you. I believe I have isolated the issue on AdBlock. I resolved it by clicking the AdBlock logo on my Foxfire browser when on the Facebook Messages page and selecting "Report Issue on This Page". AdBlock identified two filters that might be the issue. I disabled the first one but it was disabling the second filter recommended by AdBlock that did the trick. It has to do with blocking the "Sponsored" ads. Luckily, the resolution leaves the word "Sponsored" but still blocks all the ad images. Yipee - good for AdBlock to have this resolution device. I hope that works for the others affected.