A suite of tools for developing Fathom rulesets within Firefox:

Corpus collection and labeling tools (which are likely all you will need)
An Evaluator which can help you drop into the JS debugger inside your ruleset
A Vectorizer, which you can ignore. (It persists, for now, as an optional manual alternative to simply letting fathom-train and other tools take care of vectorization automatically.)

For most use cases, it's better to run FathomFox from the commandline rather than installing it through the web. See Fathom's installation page for instructions. Full Documentation See the Fathom docs. Running FathomFox from a Source Checkout This is necessary only if you are developing FathomFox itself.

Clone the Fathom repository.
From within the checkout, inside the fathom_fox folder, install dependencies: yarn run build.
Run a clean copy of Firefox with FathomFox installed: yarn run browser.
Run yarn run watch in a separate terminal. This will keep your running copy of FathomFox up to date as you edit your ruleset.

Thanks Thanks to Treora for his excellent freeze-dry library!