Probably possible, but the changes would be gone with the next Firefox update. The file where those js files are contained is replaced with every Firefox update.

That's fine, I tend not to update that specific version of the browser, this is for testing purposes.. Could you also give me steps on how to make changes in my local copy of firefox? Thanks

With a XUL based legacy extension it would be easy to override this file, but you can no longer get legacy extensions signed via the Add-ons website.

• https://developer.mozilla.org/en-US/docs/Chrome_Registration#override

Otherwise you would have to edit the file in the omni.ja archive (not recommended).

• https://developer.mozilla.org/en-US/docs/Mozilla/About_omni.ja_(formerly_omni.jar)
• https://dxr.mozilla.org/mozilla-release/source/testing/marionette/action.js

Thanks!!