Some months ago I could delete all cookies on Firefox.
On the tabs Edit - Preferences - Privacy & Security - Cookies and Site Data, the cookies would be set to zero.
If I closed down Firefox and re-started the browser the cookies would remain at zero.

However recently if I do the same operation Firefox will re-start with a minimum number of cookies.
How can I configure Firefox so that there will be no permanent cookies?

Just above that setting you have the option to Send websites a "Do Not Track" etc.
Just above that you will see the Enhanced Tracking Protection likely has a radio button selected for "Standard", hit the drop down for section "Custom" and have a look at what options you have regarding what you want and put the radio button on "Custom" not "Standard".

Prefs => Privacy & Security => Cookies and site data => Delete cookies and site data when Firefox is closed.

Thank you. I already have the 'Browser Privacy' section set to custom.
Unfortunately despite deleting all cookies and reloading Firefox - there is always a set number of cookies already on the browser.

I'm wondering why my configuration of FF doesn't delete cookies.

This part is ticked but is always faded out on firefox browser.
How can I get this option activated?

heathcliff36 - in your OP you say "Some months ago I could delete all cookies on Firefox".
Would it be right to assume that you have since up-dated Fox?

In my experience an u/dated Fox is not necessarily a better Fox.

A quick search suggest not to run Fox in permanent Private Browsing mode, although that appears to be from few years ago.

And... "This part is ticked but is always faded out on firefox browser."
I have never encountered that. Maybe a remove and re-install will fix it?

I simply download Firefox upgrades from the debian repo.

I have made some alterations to about:config to prevent webRTC leaks.

I have attempted this but for some reason Firefox didn't fully uninstall.
The problem with Firefox is you have to uninstall all dependencies as well as your 'profiles' directory in /home.

What are the commands for an effective uninstall and re-install for Firefox?

This is probably me being cautious, but I generally wait until an updated Fox is shown as an upgrade in Synaptic and even then wait a while before doing the upgrade. Not what the powers that be would advise, but I'm tired of continual changes that often appear to be for changes sake. All a bit Microsoft - the consensus seems to be "whatever you do, don't let them get comfortable..."

"To back up your profile, first close Firefox if it is open and then copy the profile folder to another location." (from a quick search) so that should be OK - maybe store on an external thumb drive, just in case?

Others may advise against using Synaptic, but if it's set for 'complete removal' in Prefs, it should be OK. At least it tells you first what will be removed, so you can always decline if necessary. Once you locate Fox in the Installed list, there is also the option to 'reinstall' - but my inclination is that removal and fresh install would be better.

Looks like a person has to do a lot just to avoid cookies! :-)

Hope it works out.

The problem with saving the profiles directory in /home is that cookies and even 'evercookies' may be stored in them.

That's interesting - I didn't know Synaptic had a 'complete removal' function.
I'm just wondering if this function will completely remove all the dependencies and the directories existing in the /home directory.

Good point. I think this is the better option.

Maybe "support.Mozilla" is the place to check for the correct procedure?
Some prior knowledge wouldn't hurt, and may help in avoiding a Catch-22.

It's the first place I looked.
It didn't help.

Really? Surprised.

Looks like you put up with the cookies, or take a roll of the dice.

The puzzling part is in post #5 where you say the Privacy & Security prefs are greyed out, which leads me to think a fresh install would help.

Your original post was only yesterday, so maybe wait a couple more days to see if anyone else can help with this or, you could browse the Debian forums and see if there's anything there on how to resolve this irritation.

That shouldn't be.
Which distro, how did you install firefox, which version?

I have Debian 10
I think I installed firefox-esr with:
It's the 78.6.1esr version currently.

On the debian page for firefox on Buster 10 it refers to:
Is that the specific package name I should've used?
I thought it was just 'firefox-esr' for all Debian distros.

The name is firefox-esr, the version is 78.6.1esr, the Debian release is 1~deb10u. Most of the time the last part (release) is irrelevant to browser behavior.

Have you set Enhanced Tracking Protection to Strict? This may affect your ability to control cookies.

OTOH, I also have Firefox 78.6.1esr with ETP set to Strict, and the "Delete cookies when closed" checkbox is not grayed out.

Anyway, going to about:config and searching for privacy there should show in bold all privacy settings that are changed from their defaults.