I can only offer this Clear cookies and site data in Firefox.

But I am guessing you already have tried, I am at a loss as to why they are not deleting.

Maybe try using the Diagnose Firefox issues using Troubleshoot Mode.

Hello DPMR, I tried on that way, but when I go to show all the cookies en try to delete a single or all the cookies the button dos not delete any cookie. When I go to history and delete this the cookies are gone. But when a start FF than they are back. The same this happens when I clear the computer whit CCleaner, firt they are gone and after restarting FF they are back. The cookies are as far I can see from advertising companies who are following my surfing. The add-on Better privacy dos not give any chance. On a other computer whit the same FF and add-ons there is no problem, Cookies are deleteble on the normal way.

Is there a way to delete the cookies on a way that they do not come back?

I am not sure what to say, I have no issue with deleting cookies at all.

Maybe when you start in safe mode you should disable all the Add-ons.

Then try to delete the cookies.

Hello DPMR, I found the problem... I started FF in save mode and disabled all the add-ons, than I enabled them piece for piece, and checked the cookies. The result was that the add-on Privacy choice Trackerblock has written 300 400 Opt-Out Cookies??? in FF to prevent following af advertising firms. They are looking as normal cookies, but they are not as far as I understand.

Thank you for your help and interest.

Great going Janw.  :)

Glad to have helped.

Please mark this thread as solved so others with the same issue will know you figured it out.