See also:

• [/questions/1109155]

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, cookies, passwords, cache, history, 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 the involved files.

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

Thanks, but unfortunately that did not work, as the site is not in my History (which I cleared, and also I do not remember history anyway). So there was no site to "forget".

Do you have any other suggestions? Or should I just re-install Firefox perhaps?

That message is generated by a filter protecting the server. It seems to be a service/device used by many sites. The filter may be reacting to something unusual about your connection, but in other threads involving other sites, users reported it cleared up by itself after a few hours, perhaps indicating that a range of addresses was blocked temporarily to stop an attack and it also affected innocent users.

Reinstalling Firefox probably won't help. If you want to see what it would be like without your Firefox settings, you can try this:

Create a new Firefox profile

A new profile will have your system-installed plugins (e.g., Flash) and extensions (e.g., security suite toolbars), but no themes, other extensions, or other customizations. It also should have completely fresh settings databases and a fresh cache folder.

Exit Firefox and start up in the Profile Manager using Start > search box (or Run):

firefox.exe -P

Don't delete anything here!

Any time you want to switch profiles, exit Firefox and return to this dialog.

Click the Create Profile button, assign a name like TEST2016, and skip the option to relocate the profile folder. After creating the profile, select it and start Firefox in that profile.

Any difference?

When returning to the Profile Manager, you might be tempted to use the Delete Profile button. But... it's a bit too easy to accidentally delete your "real" profile, so I recommend resisting the temptation. If you do want to clean up later, I suggest making a backup of all your profiles first in case something were to go wrong.

Thanks for your help. I tried creating a new profile and then running Firefox, but I still receive the same error from that site.

Access Denied You don't have permission to access "http://www.asos.com/" on this server.

Reference #18.7ffd6dcb.1455166045.2111a030

Any other ideas? I guess it could be an address block which might clear in a day or two.

If it's your address, you could work around it using a private VPN service. Since the VPN stands in for your regular ISP and can track all your browsing, please be careful in testing such services.