This is a good comment, I use FF Dev and Chrome RN, the current project I'm working on is Chrome only. It's nice for context switching (docs, etc in FF, debugging & development in GC) but I was not able to do this (1y ago) on my 2013 MB Pro, it was simply to slow.
you can right click on a tab in firefox, and switch that tab to this or that context ;)
I have a 2013 Mac Book Pro with 16 GB of RAM. They all run fine at the same time.
I also have Xcode and several other apps open.
Don’t skimp on RAM. My next computer will have 32GB of RAM. It’ll extend the usefulness by years.