Hi, you did the right thing to delete it. It's a fake update designed to infect you with malware which we have seen a lot of in recent months! NEVER EVER click on anything like that! Please see the article - I found a fake Firefox update.

If you are in any doubt about the authenticity of update pop-ups etc., it's always best to ignore them and check for genuine updates 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.

We are trying to find a permanent fix, but it's proving very difficult. A possible workaround which seems to be quite effective, is to install an ad-blocker, such as -

https://addons.mozilla.org/en-US/firefox/addon/ublock-origin

Or, but this may be less effective - https://addons.mozilla.org/en-US/firefox/addon/adblock-plus

nisacatbo said

...with an official-looking Firefox page showing congratulations your update is complete and another pop-up saying to download a file: firefox-patch.js from iengibarclaysstockbrockers.com The website doesn't appear to have anything to do with Mozilla or Firefox. The thing that made me question it though was appearing on the third tab instead of the last tab, where it usually does.

The so called official looking page was made to distract inexperienced Windows and or Firefox users (on Windows) from the that fact they are on a random weird name site and not on a *.mozilla.org url. It's good you suspected it was not real. This disposable sites fake update scam has existed since as early as May but has not been mentioned as much in forums in the past month