Could you describe the context in a little more detail. Is this a pop-up message or does it replace the page you want to access? Or did you find it in Firefox's Error Console?

Generally speaking, when you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

1. Bypass Firefox's Cache

Use Ctrl+Shift+r to reload the page fresh from the server. Any difference? If not...

2. Clear Firefox's Cache Completely

orange Firefox button or Tools menu > Options > Advanced

On the Network mini-tab > Cached Web Content : "Clear Now"

3. If needed, remove the site's cookies in this dialog

While viewing a page on the site, right-click and choose View Page Info > Security > "View Cookies"

Then try reloading the page. Does that help?

Thank u for answering. I will try your suggestion. Yes, the "403 error" page does replace the page (Pinterest) site.