Bug 253696

ModifyHeaders in declarativeNetRequest  does not work for fetch requests from the extension.
This might be since the extension does not have permission over the safari-extension://GUID/ origin which is reset on every browser launch.

Expected behaviour:
The extension should have permissions for its own origin by default and should be able to modify headers for requests initiated from its own background page.

As such adding a rule like this should work:

browser.declarativeNetRequest.updateDynamicRules({
   addRules: [{
      "id": 1, 
      "priority": 1, 
      "action": {
         "type": "modifyHeaders", 
         "requestHeaders": [{"header": "origin", "operation": "set", "value": "https://www.bar.com"}]
      }, 
      "condition": {
         "urlFilter": "http://foo.com", // Any URL that the extension has permissions for
         "resourceTypes": ["main_frame", "xmlhttprequest"]
      }
   }],
})


Current behaviour:
The rule currently does not work