Totally agreed! This is one of the major strengths of Reddit: The removal of a post doesn't lead to the destruction of the entire comment history.
This is also something I don't like about YouTube. If a video disappears, it takes the comment history with it.
and if you look for it on about:config, it’s enabled
The about:config setting has been placebo since version 48. It doesn't affect anything.
Firefox 48: (Pushed from Firefox 46). Release and Beta versions of Firefox for Desktop will not allow unsigned extensions to be installed, with no override. Firefox for Android will enforce add-on signing, and will retain a preference — which will be removed in a future release — to allow the user to disable signing enforcement.
Also vote here: https://www.change.org/p/no-add-on-signing-in-firefox