For the last five days I've been unable to login to a particular site unless I clear the cache EVERY time I want to login.

And did they provide any trace of what kind of "an error" actually occurred under the hood to help understand the reason for failing the login flow?

Hi jbr Thanks for trying to help. Short answer .................... no. They did suggest it could have been a conflict with a browser extension. I disabled them all, but that made no difference, but why would it, they've been on my browser for ages.

Simon

That's unfortunately the argument every single time an addon causes breakage. The web changes every minute, and your addons and their content filtering definitions are also updating in the background all the time, to follow suit. So that's the part which is sorta outside of your control unfortunately.

Did you attempt the same login using 1.) a new Private Window, or 2.) restarting into Troubleshoot Mode (from Help menu) to see how that works when all extensions are unloaded?

If you don't have any luck that way, there are privacy settings that could e.g. disallow any site data getting stored for given domain that you can try setting instead of your manual clearing — if the site session can work even with such exceptions set.

I've just tried to login using a new Private Window and had no issues at all. Great suggestion, thanks for that, but how can I sort the issue using a regular Firefox window?

Check Block websites from storing cookies and site data in Firefox for the general functionality — you might need to fiddle with the settings for a bit to see which a) works for the site, and b) doesn't break its logins;)) The same/similar interface is in Settings › Privacy and security › Browsing data › Manage exceptions — where you can set the domain which won't be allowed to persist any data — which might be the band–aid fix for your use–case.