You can create an exception by visiting the forum, press Ctrl+I > Permissions > Set cookies > Allow.

Ctrl+I did not give me a choice of permissions. I am on linux, xenial64 puppy linux. Could this be why.

Toggling "Delete cookies and site data when Firefox is closed" makes changes to the "Clear history when Firefox closes" settings and those settings prevail.

Create a cookie allow exception with the proper protocol (https:// or http://) to make a website remember you

Make sure to keep the "Site settings".

• clearing "Site settings" clears exceptions for cookies, images, pop-up windows, and software installation and exceptions for passwords and other website specific data

• Settings -> Privacy & Security
  Cookies and Site Data: "Manage Exceptions"
• Settings -> Privacy & Security
  Firefox will: "Use custom settings for history":
  [X] "Clear history when Firefox closes" -> Settings

Detail like websites remembering you and log you in automatically is stored in cookies, so make sure you aren't clearing important cookies.

Try using the flatpak version.