[Liftyee]

I needed exactly this sort of tool for a reverse-engineering project! I was so invested I returned here to write this comment... then spotted the other comments about "no Firefox support". Indeed, visually broken "Browser Not Supported" popup appears.

Darn. Disappointing. Guess I will have to keep looking.

Also... it doesn't look open-source and the comments about file access are valid. The functionality listed is completely possible as a browser-based local app with no server functionality.

Other webapps (https://falstad.com/circuit/) seem to be able to open a file picker in Firefox just fine. Saving is just via downloading to the Downloads folder, but the functionality is not impossible.