Further to my original post, I can sign in using other browser such as Safari.

That message is characteristic of a web application firewall blocking access to the site. This can be triggered by anything the firewall considers suspicious about the request; it is difficult to know the exact problem.

Sometimes a block may be based on your IP address. If you can connect using another browser on the same machine, then it's probably not that, unless Firefox is using a proxy or VPN and Safari isn't.

Otherwise, you might clear Firefox's web cache and the cookies for costco,ca on the Preferences page and see whether that helps:

(1) Clear Firefox's Cache

See: How to clear the Firefox cache (in Clear Data, only check the box for web cache, not all cookies and site data)

If you have a large hard drive, this might take a few minutes.

(2) Remove Site Cookies

Click the Manage Data button on the Preferences page, Privacy & Security panel. Type costco into the search box and pause while Firefox filters the list. Then select the Costco servers (Command+click to multi-select) and Remove Selected and Save Changes.

Then try visit the site again. Any improvement?

Thanks for your advice jscher2000. However, the problem still exists.

You can remove all data stored in Firefox for 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").

Using "Forget About This Site" will remove all data stored in Firefox for this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data for 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 for that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox