Hi Can you please post the error message Firefox gives you?

Something's gone wrong We may be having trouble with your connection. Connecting your device to Wi-Fi could help fix the issue or try again later. If you were using an app, close this page. Otherwise, start over.

I've tried disabling my antivirus, that didn't work either. It is just focused on this one computer as I can open it on Firefox on another computer.

Try deleting Cookies and Site Data. Go to Firefox's Settings > Cookies and Site Data > Click on the button: "Clear Data...", then choose "Everything" and mark all checkboxes and click the "Clear" button. Close and open Firefox again and retry.

I did that right after the problem started. No luck.

Have you tried turning your Wifi on and off? Try this in combination with cookie deletion. I was reading in other forums that this might be a problem on ATT's part. Look if you can sign off this device with problems from your ATT's account from other device.

Honestly, I can't see where this would make a difference. You mean on and off on the computer? Or the modem?

On both. Something similar happened to me before and restarting both Wifi and modem helped but I'm in a different OS and have Firefox installed as a Snap so I'm not sure.

I get that error on certain AT&T pages. Is it because I'm not an AT&T customer?

What if you use the Email link hosted by Yahoo:

https://currently.att.yahoo.com/

That's how I usually go in to sign in. And it's only on this computer which is confusing. I can sign on easily on another which doesn't make sense.

Thank you for your suggestions.

I don't know why you would get AT&T's error page if you are starting on Yahoo's site. I never had an SBC Global account, so I don't know how they interact.

General advice:

When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Clear Firefox's Cache

See: How to clear the Firefox cache (only select "Temporary cached files and pages", don't clear other kinds of data)

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that. Firefox will ask you to confirm; go ahead and confirm.

Then try reloading the page. Does that help?

Nope, I just tried that. I thought it was something with ATT as they have had problems before. Not this time according to them. Also contacted my anti-virus provider to see if they had made any changes, and not them. I contacted Mozilla support, and they couldn't do anything about email, although it appears to be a Mozilla problem on this device. This happened before, and it was a bug in Mozilla's software.

Frustrating to say the least...