Make sure you are not blocking content.

Diagnose Firefox issues using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop also see https://blog.mozilla.org/security/2021/03/23/introducing-smartblock/

https://support.mozilla.org/en-US/kb/smartblock-enhanced-tracking-protection

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connection certificates and send their own.

https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites

https://support.mozilla.org/en-US/kb/secure-connection-failed-firefox-did-not-connect

https://support.mozilla.org/en-US/kb/websites-dont-load-troubleshoot-and-fix-errors

What do the security warning codes mean

Fred -

Thanks for the prompt reply! Unfortunately, the behavior is exactly the same in safe mode, and McAfee is the only security software I'm running. There are no error messages other than the "... wires crossed... try again" I mentioned in my post. Flushing the caches and cookies was the first thing I tried, and as I said, the site works fine using Firefox on my Mac, as well as with Chrome and Edge on Windows and Safari on my iPhone. Really strange...

Delete browsing, search and download history on Firefox {web link}

Remove a single website from your history {web link}

Open the History Manager <Control> (Mac=<Command>) <Shift> H. In the search bar, enter the name of the site. Right-click on one of the listings and select Forget About This Site. This should remove all information, including any site settings And Passwords.

Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

Starting Any Computer In Safe Mode (web link) Free Online Encyclopedia

https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/

https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/ +++++++++++++++++++++++++++++++++++++++++++++++ If the problem goes away with the computer in Safe Mode, the problem is with one of the other programs on the system.

As a test, disable your protection programs.

Fred -

Thanks again for more suggestions - all tried and still the same result. This is most puzzling since the site works fine with the other browsers on Windows and iOS, and with Firefox on Mac.

Regards,

Howard

I called for more help.

Excellent Fred - Thank You! Looks like you have additional insights into the support org that I don't have.

I just compared the settings on my Mac and Windows FireFox browsers (both at 92.0.1 (64-bit), and with the exception of a few that weren't relevant, they were identical. I changed some settings to make them consistent on both machines, and the MacAir still works and the Windows version does not.

Really curious...

Regards,

Howard

Use these links to create a new profile. Use this new profile as is. Is the problem still there?

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Type about:profiles<enter> in the address bar.

Fred -

Thanks for these suggestions! I am now able to log in to the t-mobile site. As I thought, the problem was not a corrupted profile, but creating a new profile allowed my to debug the issue from a known good configuration.

The new profile allowed me to successfully log into the site, after which I gradually started adding information and extensions from the original profile, verifying correct operation at each step. The last thing I did was update the default settings, and that uncovered the issue. When I selected "Enable HTTPS-Only Mode in all windows", the login process failed. Even though the t-mobile site is an https site as would be expected, their user id and password verification processes invoke a variety of scripts and additional websites. Apparently, one or more of the sites running these processes do not accept https. When accessing standalone http only sites with this mode selected, Firefox will display a dialogue to ask if you want to proceed with http, but if that message was created it was never visible, and those processes simply timed out causing the log in failure.

I have now reverted to my original profile, selected "Don’t enable HTTPS-Only Mode", and everything is working as it should. Thanks again for your timely responses and for continuing to provide new information. Have a great weekend!

Regards,

Howard

That was very good work. Well Done. Please flag your last post as Solved Problem as this can help others with similar problems. Go to that post and click the 'Solved' button to its right.

Thanks Fred! I can't mark my own reply as "solved problem", but your post is now immediately under my original post as the "chosen solution", and anyone who needs more context can read through our thread.

Take care!