The real meaning of “faire long feu” is “to fail”. “Ne pas faire long feu”, has an unrelated meaning, and means “to not last/stay long”: “Je n’ai pas fait long feu à cette soirée”. The source above makes it unrelated, but it is possible that people just lost sight of the original meaning (the metal cartridge was invented a couple of centuries ago…).

There is a recurring ambiguity around these expressions.

The set expression ne pas faire long feu (to occur quickly) is generally well known. The issue is when people want to negate it.

Ne pas ne pas faire long feu is logically simplified to just faire long feu and expected to mean “to last long, to take a long time to occur”. This clashes with the other, less known, set expression identically written faire long feu, which means “to eventually fail to achieve success”.

A good example of such a confusion was a reply by a former French Prime Minister (Jean-Marc Ayrault) to a Le Parisien newspaper reader question where faire long feu clearly means to both parties the opposite of ne pas faire long feu:

Marc Ichbia: — Pensez-vous faire long feu à Matignon ?

Jean-Marc Ayrault: — Oui, je pense. L’action que je mène au gouvernement ne peut que s’inscrire dans la durée. Ce ne sont pas les yoyos de politique publique qui pourront résoudre les problèmes du pays. Ni en quatre jours que l’on va régler les problèmes.