Firefox on Android 16, it sucks the battery up in background mode (even with permission to use background turned off).

If I manually close it no issue.

I believe you.

I also do not, and have never, experienced this. I've been using Pixel phones since the 3a in 2019/2020.

doesn't happen in my case (OnePlus 15/Android 16, Firefox background usage allowed in some "smart" mode whatever that means, doesn't suck the battery in background although it's on most of the time).