Just checked the Wife's computer.
Just updated the computer in the barn (to Firefox 78.1.0esr running on kernel 5,4,54). Firefox 78.0.2esr had been running non-stop on kernel 5.4.51 since July 10th. It had been rather heavily used in those three weeks of uptime. There was no sign of memory being gobbled up &etc.
The dev box here in the office that I use to keep an eye on things like this has similarly seen none of the issues that you describe. Bleeding edge kernel & Firefox, updated sometimes daily.
I suspect there is something else wrong with your computer.