If you see consistent script errors on unrelated sites, it could be caused by an add-on. You might simply disable all non-essential add-ons, restart Firefox, and see whether the errors go away.

orange Firefox button or classic Tools menu > Add-ons

Check both the Extensions category and the Plugins category.

Another standard diagnostic is to try Firefox's Safe Mode (Diagnose Firefox issues using Troubleshoot Mode).

First, I recommend backing up your Firefox settings in case something goes wrong. See Back up and restore information in Firefox profiles. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

Next, restart Firefox in Firefox's Safe Mode using

Help > Restart with Add-ons Disabled

In the Safe Mode dialog, do not check any boxes, just click "Continue in Safe Mode."

Any luck?

Thanks for taking time to address my problem. I wish there was a better way to express my feelings for your kindness.

Since my first reporting, now the Google Calendar issue has gone away -- meaning, I do not have any problems with my Google Calendar any more. And, I do not know of anything that I did to help resolve that.

However, I continue to keep getting the error: this.container is null. This error comes when I start the FireFox and a couple other sites that I usually visit. Any thoughts?

Did you try to disable all extensions?

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

No extensions are shown. I have removed ALL add-ons before.