I go to Norton, sign in, then go to another sigh-in button and the screen just flashes. Click it several times and just see the screen flash.

Is logging into their site a step to complete the installation of Norton on Windows, or to install a Norton add-on into Firefox from their site?

Either way, could you double-check a possible content blocking issue: Firefox's Tracking Protection feature, and extensions that counter ads and tracking, may break websites that embed third party content (meaning, from a secondary server).

(A) The shield icon toward the left end of the address bar usually turns a bit purplish when content is blocked. Click the icon to learn more or make an exception. See: Enhanced Tracking Protection in Firefox for desktop.

(B) Extensions such as Adblock Plus, Blur, Disconnect, DuckDuckGo Privacy Essentials, Ghostery, NoScript, Privacy Badger, uBlock Origin or uMatrix usually provide a toolbar button to manage blocked content in a page. There may or may not be a number on the icon indicating the number of blocked items; you sometimes need to click the button to see what's going on and test whether you need to make an exception for this site.

Any effect from that?

I had actually installed FireFox and then Thunderbird, plus a few other non-internet programs. Then I remembered that the first thing I should have done after installing Firefox was Norton.

I got a suggestion to try another browser, so I downloaded Chrome and tried it, and it worked ok.

(I don't use chrome for anything else, I have been using Firefox for - it seems years and years - and I'm perfectly happy with it)

But this information is good for me to know as I will be setting up some ad blocking and the info on extensions is nice to have. Thank you!

I'll go ahead and marked this as being solved.