NEVER EVER, NEVER, NEVER, NEVER EVER click on a pop-up. A pop up can do anything and you have absolutely no idea who sent it or what it will do.
If you want to update Firefox, just go to the Tools menu and click on Options. Then go about half way down the General page there to see your options for updating. I use the automatic update so I am always up to date.
My first sentence above applies to ANY pop-up. I get constant requests to update things with pop-ups. I NEVER click on anything except the X to close those windows. Always update any program from the menus IN THAT PROGRAM or from their web site WHICH YOU REACHED BY ENTERING THE ADDRESS YOURSELF. NEVER FROM A POP-UP OR A LINK.
With just a few keystrokes I, a real internet amateur, can copy any page or pop-up that appears on my screen - a perfect copy. I don't know how to create a false pop-up from there, but you can bet that the bad apples out there can. A pop-up that looks 100% normal can wipe your hard drive or scramble your data so you have to pay a ransom to get it back.
NEVER EVER, NEVER, NEVER, NEVER EVER click on a pop-up.
And NO, I am not getting that. But if I did, I would totally ignore it.