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...

O.K., here's a new twist.

This has been working for the last few days.

I can log in on Mozilla but in a Private Window only. Does anyone have any idea why this works and not a regular window?

I can log in on Mozilla but in a Private Window only. Does anyone have any idea why this works and not a regular window?

I don't know about that site in particular, but here are some general possibilities:

(1) Private windows do not shared previously saved files in the web content cache or previously saved cookies with regular windows. It's a fresh start every time you close all private windows.

(2) Extensions require specific approval to run in private windows, so many users have different content blockers running in regular and private windows. More info in the following article: Extensions in Private Browsing.

(3) Private windows sometimes have stricter Tracking Protection than regular windows. But it's difficult to see how that would help this situation.

As @jscher2000 said, maybe that particular website is having a conflict with one of your installed extensions. According to the settings you chose when installing a particular extension, they may or may not be running on Private Browsing so you may find the extension that's having conflict by disabling each extension and visiting the site again (remember to force refresh using Ctrl + F5) to know if there's a problem with any particular extension.

I have AT&T and have been having the same problem. It started out on my desktop, now it's my laptop also. I can log on with Microsoft Edge or Chrome, but not Firefox. This may be a problem with Firefox and AT&T. I have no idea but it's very frustrating. I've tried all the things suggested here. No luck.

In another thread, a user reported that they had to disable (or make site exceptions) in AdBlock Plus and DuckDuckGo Privacy Essentials to get sign-in working again. That was while viewing the AT&T error page. If you use either of those extensions and do not see their toolbar buttons (which allow you to make a site exception), check the Extensions button drop-down. See: Manage your extensions using the extensions button in the toolbar.