doesn't firefox already has an internal json viewer?

This extension is not as good as the json viewer that's built into Firefox since v53. See https://developer.mozilla.org/en-US/docs/Tools/JSON_viewer

The chrome version automatically formats json even if the page/file extension isn't "json". The Firefox version should do so too.

使用确实不错

not working on Firefox 66. JSON files still are only offered to be downloaded

Firefox already has excellent built-in support for viewing JSON files. (Currently, I am using Firefox 65.) This add-on is a serious regression in comparison.