Don't know if it makes a difference, but forgot to mention that gmail indicates a possible problem with javascript being disabled. However, about:config shows javascript.enabled as "True."

You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.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/Options/Preferences -> Advanced -> Network -> Cached Web Content: "Clear Now"

"Remove the cookies" from websites that cause problems.

• Firefox/Options/Preferences -> 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 or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• Do NOT click the "Refresh Firefox" button on the Safe Mode start window

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

Thank you, cor-el!

Reloading the page, clearing the cache and removing the cookies did NOT solve the problem, BUT Tools--Add-Ons--Extensions did SOLVE the problem. For some reason, javascript was blacklisted for gmail.

Is YesScript a product of a recent Firefox update, or a malicious program that should be removed?

YesScript and NoScript are extensions that you can install to enhance Firefox with new features. It is up to you to install extensions in case you need these features. YesScript allows JavaScript on websites unless you create a block rule. NoScript does the opposite and blocks JavaScript among other things and needs an allow rule to unblock content.

If YesScript has a block rule for Gmail then you need to remove this rule.

• https://addons.mozilla.org/firefox/addon/yesscript/

YesScript lets you make a blacklist of sites that aren't allowed to run JavaScript. Use YesScript on sites that annoy you or hog your system resources. One click to the icon in the status bar turns scripts on or off for the current site.