How do inject a script into a website *before* the first script on this website is able to run?

[Manuel Reimer]

On 08/15/2017 06:07 PM, The 8472 wrote:
> You really should not load things via a <script> tag when you actually want to run it as content-script. Those are separate security scopes.[0]

I don't want to run it as content-script.
I want to implement some functionality available to page javascript 
without worrying about security problems. That's why I just inject the 
code into a <script> tag. This way this piece of code has the same 
privileges as the page javascript itself.

I now placed my "page code" into a separate file and use the array to 
load both.