Yes, it is a malware.

I found a fake Firefox update

. Whenever you get a message / popup that software / files need to be updated;

DO NOT USE ANY OF THE PROVIDED LINKS

While this may be a legitimate message, it could also be Malware or a Virus. Anytime you want or need to check for upgrades, go to the website of the True Owner of the program in question. For example, to check out Firefox, go to https://www.mozilla.org {web link}

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}

For almost a year, an epidemic of Fake Update Notices have been popping up all over the place. https://support.mozilla.org/en-US/kb/forum-response-i-found-fake-firefox-update

seagull504 said

Without rning the page I had open (e.mails) was replaced by a very purple page with a large 'Firefox' logo and "Critical Update"

Was this opened instead of, or in a separate window?

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.

Go to the Mozilla Add-ons Web Page {web Link} (There’s a lot of good stuff here) and search for a good ad / pop-up blocker.

Thank you all for your comments. They confirm what i thought.

mesage re-appeared; the address line is: https://uuquumeilenrechner.net/7741455388289/5e90948c72dc854046268855b75f7673/1bfa3d8cf9cceecea37dd19b49d138c1.html