i try it usually once a year to confirm there is no progress and experienced this few weeks ago, loading and rendering pages it's slowest compared to Chrome, CAF browsers or Brave, heck Firefox doesn't even have pull down to refresh which is showstopper, add-ons are nice but most of the users don't care and i can block ads just with adaway
as for desktop, again unable to handle more tabs properly, freezing and unresponsive while no such problems on Chrome
i dunno maybe Chrome devs test their browser on actual real life desktops and mobiles, while Firefox devs think everyone has flagship and computer with 16gb of RAM?
TLDR: Firefox is significantly (not slightly as article honey talk) slower on mobile and freezing and unresponsive on desktop compared to Chrome and real users notice this, they don't care about add-ons