It isn’t really, just marketing fluff. Foxify[1] allowed Firefox to install Chrome extensions almost a decade ago and then I ported that to Waterfox after privileged extension support was removed from Firefox 56.

Currently still in Waterfox stable but figured there’s not actual much advantage to supporting Chrome extensions as Firefox ones are more featureful, so have removed it in the final version based on ESR 153 - albeit would be trivial to add support back in.

[1] https://github.com/bnpoirier/foxify