Short answer: Safari 14 supports the same standard for extensions that Firefox and Chrome do, with the addition of some security issues the other browsers don't seem to care about, including not implementing certain features to hurt privacy and security.
Many authors have used Apple's utility to convert their extensions for use in Safari.
Many authors have used Apple's utility to convert their extensions for use in Safari.
It's pretty straightforward; here's an example: https://ngyikp.com/safari-14-webextensions/
The author will likely need to make a few tweaks and changes; it shouldn't be that hard.