I have tried openSCAD and FreeCAD, they are both good in their own right, but utimatly they also both have very steep learning curves. I suppose Blender can also be used for CAD but I have no experience with it. I just want to quickly design some parts for 3D printing as a hobby and don’t feel like spending hundreds of hours learning those tools. I am current using Onshape.com, it works well on Linux/Firefox, suits my needs and free to use with some limitations. But it is proprietary :(