How/when does this show up?

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

Did that warning screen show up with phone support numbers? If so . . . .

. Whenever you get a message / popup like that;

DO NOT USE ANY OF THE PROVIDED LINKS OR ANY PHONE NUMBERS ! !

Such messages are only Spam that could lead you to Fake Support. Not only can they damage or hack your system, they charge you to do so.

You can report such a site at; Google Report Phishing Page {web link} which is the same when done while on site by going to Help > Report Web Forgery

Help us safeguard Mozilla’s trademarks by reporting misuse {web link}

Thank's for the Tipps. Virus scan I have done, without result. All add-ons are exclusively from Mozilla and have been running for quite some time without any problems. Meanwhile, the message no longer appears, so I can not determine where the cause was. Nevertheless, many thanks for the quick answer.

Avast was having a false positive on that URL. Avast is a pretty low quality Anti-virus, I suggest uninstalling it and using Microsoft Security essentials and a monthly malwarebytes scan.

Tyler Downer said

Avast was having a false positive on that URL. Avast is a pretty low quality Anti-virus, I suggest uninstalling it and using Microsoft Security essentials and a monthly malwarebytes scan.

Uhps, ok and thanks