Selenium has now supported Chrome Devtools Protocol (CDP) API, so , it is really easy to execute a script on every page load. Here is an example code for that:

driver.execute_cdp_cmd('Page.addScriptToEvaluateOnNewDocument', {'source': 'alert("Hooray! I did it!")'})

And it will execute that script for EVERY page load. More information about this can be found at:

• Selenium documentation: https://www.selenium.dev/documentation/en/support_packages/chrome_devtools/
• Chrome Devtools Protocol documentation: https://chromedevtools.github.io/devtools-protocol/tot/Page/#method-addScriptToEvaluateOnNewDocument

Since version 1.0.9, selenium-wire has gained the functionality to modify responses to requests. Below is an example of this functionality to inject a script into a page before it reaches a webbrowser.

import os
from seleniumwire import webdriver
from gzip import compress, decompress
from urllib.parse import urlparse

from lxml import html
from lxml.etree import ParserError
from lxml.html import builder

script_elem_to_inject = builder.SCRIPT('alert("injected")')

def inject(req, req_body, res, res_body):
    # various checks to make sure we're only injecting the script on appropriate responses
    # we check that the content type is HTML, that the status code is 200, and that the encoding is gzip
    if res.headers.get_content_subtype() != 'html' or res.status != 200 or res.getheader('Content-Encoding') != 'gzip':
        return None
    try:
        parsed_html = html.fromstring(decompress(res_body))
    except ParserError:
        return None
    try:
        parsed_html.head.insert(0, script_elem_to_inject)
    except IndexError: # no head element
        return None
    return compress(html.tostring(parsed_html))

drv = webdriver.Firefox(seleniumwire_options={'custom_response_handler': inject})
drv.header_overrides = {'Accept-Encoding': 'gzip'} # ensure we only get gzip encoded responses

Another way in general to control a browser remotely and be able to inject a script before the pages content loads would be to use a library based on a separate protocol entirely, eg: Chrome DevTools Protocol. The most fully featured I know of is playwright

If you want to inject something into the html of a page before it gets parsed and executed by the browser I would suggest that you use a proxy such as Mitmproxy.