As I mentioned, it is a research project, so I would not expect production ready code or multi-browser support. They are indeed not supporting the old version, as they are rewriting almost everything from scratch.

Last time I had contact with them, they were exploring using Plasmo [1] as building block for the extension, instead of doing everything vanilla as they did in the 1st version, which would offer cross-browser support out of the box.

But meanwhile, you can check the code [2] and add the FF manifest yourself to try it out.

[1] https://github.com/PlasmoHQ/plasmo

[2] https://github.com/Vis4Sense/HistoryMap