There is security software like Avast and Kaspersky that intercept secure connections and send their own certificate. You can check the log in the Web Console for error messages and the Network Monitor (Firefox/Tools > Web Developer) to see if content fails to load or is possibly blocked. *https://developer.mozilla.org/Tools/Web_Console *https://developer.mozilla.org/Tools/Network_Monitor Use "Command + Shift + R" to reload the page and bypass the cache to generate a fresh log. ----- Clear the cache and remove cookies from websites that cause problems. "Clear the Cache": *Firefox > Preferences > Advanced > Network > Cached Web Content: "Clear Now" "Remove Cookies" from websites that cause problems: *Firefox > Preferences > Privacy > "Use custom settings for history" > Cookies: "Show Cookies" Start Firefox in <u>[[Safe Mode|Safe Mode]]</u> to check if one of the extensions (Firefox menu button/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem. *Switch to the DEFAULT theme: Firefox menu button/Tools > Add-ons > Appearance *Do NOT click the Reset button on the Safe Mode start window *https://support.mozilla.org/kb/Safe+Mode *https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes