Depending on where your "Internet" boundary starts, you can clone the repo and run its install.sh although I think it's for really lazy people since it just downloads the GitHub release artifact anyway https://github.com/SeaQL/FireDBG.for.Rust/blob/1.81.0/instal...

I don't see that VSCode is required, they have a section dedicated to CLI usage https://github.com/SeaQL/FireDBG.for.Rust#firedbg-command-li... and even the "firedbg open" seems to just be a convenience method for file globbing https://github.com/SeaQL/FireDBG.for.Rust/blob/1.81.0/comman...

I'm a little with you that it is sus that they don't seem to have any .ts nor package.json file in that repo which would allow one to (a) see what the extension does (b) build the .vsix themselves