Disable Firefox Same Origin Policy without installing a plugin

Rather than directly answer your question, this alternative might be viable if you also have ownership of the server

Get your server to add the following response header. (+ Apply a DevOps process or piece of code to ensure only apply this code during development)

Access-Control-Allow-Origin

.. With the value of your origin domain, e.g. http://example.com or alternatively * for all domains.

After having tried to find a Firefox setting for various hours, and after having opened a bounty, I think the right answer to this question is:

At the moment of writing (March 2018), it is not possible to disable the same policy origin in Firefox by simply setting a flag.

I would personally recommend people to use Chrome instead for this kind of work, because disabling this setting is very easy, quick and doesn't involve installing third-party software.

There is a boolean in Mozilla Firefox that should allow toggling of the same origin policy called security.fileuri.strict_origin_policy.

Go to about:config in your browser and accept the risk:

Then search for security.fileuri.strict_origin_policy and double click it to toggle it to false like so:

I have not tested this but in my experience, this is the flag controlling the same origin policy.