Electron is based on chromium and I want to give non-chromium based things a fighting chance. It also feels like a memory hog and I like running slack in my browser more than as a separate app.
tl;dr nonsensical reasons that don't really stack up but I'm electron free so may as well keep it up
One of the reasons I don't switch to Firefox full time is that it (still, after many years) leaks memory.
Use Task Manager to see how much RAM Firefox is using once it's been open a while.
It is unlikely you are seeing a memory leak, it is far more likely you are seeing caching. Using spare memory is far better than not using it - and if another app needs it, FF will free it up.