この解決策はうまくいきました。Mozilla サポート フォーラムで見つけました。クレジットはすべてユーザー「cor-el」に帰属します。
You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.
Hold down the Shift key and left-click the Reload button
Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
Press "Command + Shift + R" (Mac)
Clear the Cache and remove the Cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).
"Clear the Cache":
Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"
"Remove the Cookies" from websites that cause problems.
Firefox/Tools > Options > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"
Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.
Switch to the DEFAULT theme: "3-bar" menu button/Tools > Add-ons > Appearance
Do NOT click the "Refresh Firefox" button on the Safe Mode start window