This is fairly common for extensions to be on Chrome/Firefox but not Safari, unfortunately. Firefox's WebExtension APIs are mostly code-compatible with the extension API supported by Google Chrome and Opera, with a few exceptions [1].

Safari uses its own Safari Extensions JS, so while it may be possible to share some resources (e.g. images, HTML content), the js work needs to be undertaken as a separate development project, unless you use/build a polyfill.

[1] https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Web...

The JavaScript Safari Extension API is deprecated and extensions using it do not work in Safari 13.

Safari has traditionally used a different extension API, which coupled with this smaller adoption has hindered the progress of extensions in the platform. A recent change to an app extension model has hurt the ecosystem even more, as it killed a significant portion of the already few extensions that used to exist.