A "501" error is an error on the server that can be caused by a programming error or by request not having the expected information. A "404" error is a report from the server that the page you requested was not found.

In either case, if this site has worked normally in Firefox before, I suggest the following. 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.

Alternately, you also can clear Firefox's cache completely using:

orange Firefox button (or Tools menu) > Options > Advanced

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

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

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site:

• right-click and choose View Page Info > Security > "View Cookies"
• Alt+t (open the classic Tools menu) > Page Info > Security > "View Cookies"

In the dialog that opens, you can remove the site's cookies individually.

Then try reloading the page. Does that help?

If a tab closes by itself after it launches, this could be caused by a script in the page or by a tardy pop-up or ad blocker thinking it's an unwanted page. Can you check your extensions for anything along those lines and check its Options button? Also, if you haven't updated extensions for a while, that's worth a try.

orange Firefox button (or Tools menu) > Add-ons > Extensions category

orange Firefox button (or Tools menu) > Add-ons > "gear" icon > Check for Updates

Last but not least... after an update, your security suite or firewall software sometimes blocks the changed version of Firefox. In that case, you will not get any pages at all. This is for your protection, because sometimes software changes are the result of malware. However, assuming you trust the upgrade, you may need to fix your firewall rules. Check out this article for more information: Firefox can't load websites but other browsers can.

Thanks, Simple if you know what you are doing again thanks.