JSON Beautifier & Editor от Joe Ertaba

Note: To use this JSON viewer and editor instead of the default one you need to turn the default one off as explained in:
https://developer.mozilla.org/en-US/docs/Tools/JSON_viewer

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This extension monitors JSON pages and converts them into a human-readable object. You can view, edit, search through values and keys. You can even change the format of a key or drag it into a new place. There is no need to do anything after installation. Just open a JSON page and the extension automatically converts the page into the editor mode.

As of version 0.2.0, the extension uses a dark theme if your browser's default theme is dark.

Supported mime types:
• application/json
• application/x-javascript
• application/hal+json
• application/vnd.error+json
• text/javascript
• text/x-javascript
• text/x-json
• text/plain

FEATURES
• Supports both JSON & JSONP formats
• Has light and dark syntax highlighting themes
• Supports collapsible trees
• Converts text links into clickable URLs
• Supports raw, code, and parsed JSON views
• Keeps expanded structure when refreshing the JSON page

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Change Logs:
0.1.3:
1. It is now possible to beautify local (file://) JSON files.
2. Extension now uses "persistent = false" background page (event page) to have no performance effect when it is not used.
0.1.6:
1. It is now possible to change the view mode to "Tree", "Code", and "Text" modes. The coding mode is useful to edit the raw JSON, and the text mode is useful to copy the raw data. The default view is the tree view which displays the JSON in a formatted and highlighted format.