I'm on Firefox 70.0 and do not see it doublespaced.
For a 2018 article which may apply to your version read:
The new version of FF was also reflected in this unpleasant surprise for me. Once upon a time I fixed it, now I can't find a suitable way to do it. It was certainly not a manual configuration of the userChrome.css file, as recommended here: https://www.ghacks.net/2013/11/05/firefox-reduce-line-height-list-elements-browser/.
If I do it accordingly, it doesn't work with 68.2.0esr_64bit. Does anyone know how?
thanks for any advice