Hi, we are getting a lot of reports like yours of late - it's a scam designed to infect you with malware! NEVER EVER click on anything like that.

If you are in any doubt about the authenticity of update pop-ups etc., it's always best to ignore them and check for yourself by opening the 3-bar menu > Help (question mark at bottom right) > About Firefox. Alternatively, you can press the Alt key to bring up the main menu bar > Help > About Firefox. More - Update Firefox to the latest release.

A possible workaround is to install an ad-blocker, such as -

https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/?src=ss

Or https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/?src=ss

For reference only - please don't post here - this forum is for Mozilla Support Contributors only https://support.mozilla.org/en-US/forums/contributors/712056?last=69507

If your question is resolved by this or another answer, please take a minute to let us know. Thank you.

Thanks for the reply, I hope the screenshot provided enough information to help you narrow down the cause. There were no viruses found on my computer before or after this incident, I did refresh Firefox yesterday so I didn't have my adblocker installed like I usually do. I consider myself to be a more advanced user when it comes to browsing the internet so I don't believe I did anything so careless that would have caused something like that to happen, if there's any way I can help (might not be able to do much) feel free to let me know. Thanks again!