I hate to break it to OP, but OP's metal is just not enough to run the latest and greatest softwares like KDE and Firefox. It's gonna be a rather clunky game of tradeoffs of what OP is willing to live with. Up-to-date stuff that will run on OP's metal is gonna be missing features that are important to OP, and it won't be the easy and convenient stuff that everyone knows. I've been down that road, I do have old hardware that I played with in my past.
I know what it's like to have the choice of being up-to-date, cheap, fast. Choose two, you can't have all 3.