I can't speak for the Librem 5, but I have the latest PinePhone Community Edition with PostmarketOS. It works out of the box for most things, including calls. The cameras and HDMI output aren't working right now, but fixes are coming soon for these issues (like next week).
Firefox works well with several tabs open. There are UI issues, but this is to be expected since practically no Linux desktop apps are designed for small screens. However, it's perfectly capable of running a responsive web app or two.