It may or may not be normal...

On a site with highly sensitive information, you definitely would not want to see a mixed content warning.

However, on a site like Facebook with a lot of user-generated content and materials embedded from other sites, you may see that warning from time to time if the shared content was not embedded in a secure manner.

You can try to reload the page to see whether re-fetching the content resolves the issue, but because the FB news feed loads dynamically as you scroll, that might not help on FB...

It has been going on for awhile now. I use a Macbook. Sometimes reloading helps and sometimes it does not. Like I said, it goes back and forth. Should I be worried about any malware or viruses being put into my system?

I don't think there's any indication of a virus or malware at this point. The reason for a mixed content warning is the possibility that personal data could "leak" from the secure parts of the page to an insecure site.

Thank you. I don't understand why it does it. I just wanted to make sure my computer is safe.

You can open the Web Console (Firefox/Tools > Web Developer;Ctrl+Shift+K) and check the Net log for lines with a red vertical bar and a mixed content warning icon. You can also check the Network tab for mixed content. Clear the log and reload the page to get an updated log.

• https://developer.mozilla.org/Tools/Web_Console