Yep, I check the background apps fairly regularly ('cause I'm paranoid about this battery). I'm on 14 already, strangely, I did notice a difference for the first couple of days - but back to draining quickly after that. I use Firefox as my default browser too (with ublock, yes) - I don't browse on my phone a lot.
I took my phone off charge (100%) at 17:30 last night, used WhatsApp to text some friends off and on (like, 10-20 messages total) for the next 3-4 hours and it was at 50% by 22:00, I was not using my phone, I was watching TV. It went into DnD at 22:00, and then when I got up at 07:00 it was at 25%.
So that's 75% with very light usage, with DnD for ~9 hours, in just over 12 hours.