Please explain the problem in detail. What happens? What is/are the exact error message(s) ?

Make sure you are not blocking content.

Start Firefox in Safe Mode {web link}

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

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

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}acking-protection-firefox-desktop

When I attempt to log in, I receive the attached message. I have tried clearing the cache and have used the safe mode, but still receive the same error.

Is that the whole message?

Please provide a public link (no password) that we can check out. No Personal Information Please !

Note: Some websites just don't like Firefox.

Websites say that Firefox is outdated or incompatible even though it's the latest version

https://support.mozilla.org/en-US/kb/how-reset-default-user-agent-firefox

You can check out at the browserspy website how websites see you:

http://browserspy.dk/browser.php

http://browserspy.dk/headers.php

I had the same issue a few moments ago.First I cleared my cache to no avail. Then I turned off any privacy filters or adblockers for that site. It worked after that.