What security software are you running? Windows home or pro? If pro, you should have Windows Sandbox enabled for testing. https://www.howtogeek.com/399290/how-to-use-windows-10s-new-sandbox-to-safely-test-apps/ You can also boot to safe mode with networking to test.

I'm only using Windows Defender as my security software Tried using Windows Sandbox and installed Firefox there. The problematic websites still give me the PR_CONNECT_RESET_ERROR.

What site?

One of these sites is <https://app.musicleague.com/home/> I usually just open up another browser whenever I want to access this site

Try renaming the cert9.db file in your profile folder while Firefox is not running so that a new one is generated.

I've tried renaming it, and got the newly generated one. The site still gives me the same error.

GAME OVER!  ;-))

Where did you get Firefox from? Try using another version, download and install in a different directory than your current install. https://www.mozilla.org/en-US/firefox/channel/desktop/

I've got Firefox from the Mozilla downloads, around 4 years ago.

Before downloading another version, just now. I've tried to visit the site and for some reason, it is now working properly. I've done nothing between the certificate change and today. So, I really don't know if that fixed it. [For marking which reply solved the problem]

I guess that resolves this issue I've had for months. Really appreciate the help. Thanks.