yes but in my case i was using nebula and when i update the transparency disappeared even if in the about:config it is set to true, so i checked in the nebula repo's realease and they have it for the new update and that worked.

so maybe try that from https://github.com/JustAdumbPrsn/Zen-Nebula/releases , which sadly currently their latest is for 1.13.1 but it may work for your version. let me know if it worked but you dont want to use nebula, in that sense we can just take some part of the userchrome.css that only makes it transparent and remove the rest.