I think this is not firefox issue. Try to clear cash and then retry again. Also don't forget to use the latest version of Firefox. https://www.mozilla.org/en-US/firefox/new/

ilsaggio said

Unable to log onto duckduckgo with Firefox, but no problem logging onto the same site with Chrome.

I cleared the cache and downloaded the newest version of Firefox. I'm still having the same problem. I am unable to open duckduck.com on Firefox. I can open that website immediately on Chrome, but not on Firefox.

How about this - https://duckduckgo.com/ - does that work?

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History > Show All History" or "View > Sidebar > History") or via the about:permissions page.

Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks, history, cookies, passwords, cache, and exceptions, so be cautious. If you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website then data from that website will be saved once again.