I can't log into Panda Express on Firefox, but I can log in on Google Chrome

Can you log in from multiple devices at the same time?

You could clear cookies and site data for one of the sites and see if that helps.

I cleared all history and cookies. Does not help.

The site seems to be protected by Cloudflare. They do not like privacy tweaks like anti–fingerprinting measures and such. A good test is to create a new testing profile on the side, to access the site with factory defaults — to see if it's profile–related, or anything on that connection is denied wholesale even with factory stock setup.