I've converted an earlier version to Firefox a couple of years ago, and afaik the process for Safari is similar, it only takes a few changes, replacing "chrome" with "browser" in the source code mainly.

But maintenance-wise, that's another story, it means submitting every new version to both stores, updating descriptions, screenshots etc, which takes a sizeable amount of time, and providing support for users, which is harder since I don't use Safari personally, which is why I focus on Chrome.