Why visiting the same exact link with two different browser yield different results?

With the first browser, I get "Bad gateway" no matter how many times I try to reac the page.
With second browser, no problem.

Boths browsers are firefox.

>>1534042 (OP)
The site could ban specific browsers/versions. It's a forum, ask them not us, lol

So many potential causes.
Cookies or cache. Try flushing both in the browser with the error.
Firewall. Different browser means it could be set up with different permissions.
Older browser. Needs to be up to date.
Different browser. Different extensions and plugins. Disable.
Bad VPN or proxy. Don't use.
Different device running the browsers. Different DNS. Change DNS or flush cache.

Let us know if any of that works so that others don't waste their time on a solved problem.