VegaLite.jl uses xdg-open on linux to launch the browser. See e.g. XDG MIME Applications - ArchWiki for the info needed to fix it. You probably want to use xdg-mime default firefox.desktop text/html or something like that; or maybe gnome has a tool for configuring default applications?
On my system, xdg-mime query default text/html returns firefox.desktop, and vegalite.jl works pretty much as expected.