[Bug]: NS_ERROR_PROXY_CONNECTION_REFUSED when trying to override headers in Firefox with Proxy enabled

[paveldudka]

Version

1.48.0

Steps to reproduce

In my setup I try to override "User-Agent" http header while using Firefox + HTTP Proxy
However I get NS_ERROR_PROXY_CONNECTION_REFUSED error.
Important notes:

• I verified that my HTTP Proxy works fine
• Commenting out part that is responsible for http header solves the issue
• Switching to Chromium solves the issue

My script:

import asyncio
import logging

from playwright.async_api import ProxySettings, async_playwright
logging.basicConfig(level=logging.DEBUG)


async def main():
    proxy: ProxySettings = {
        "server": "<proxy host>",
        "username": "< proxy username>",
        "password": "<proxy password>",
    }
    

    async with async_playwright() as playwright, await playwright.firefox.launch(
        headless=False,
    ) as browser:
        context = await browser.new_context(proxy=proxy)
        page = await context.new_page()
        await page.route(
            "**/*",
            lambda route: route.continue_(
                headers={
                    "User-Agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:131.0) Gecko/20100101 Firefox/131.0",
                }
            ),
        )

        await page.goto("https://bot.sannysoft.com/")

        await page.wait_for_timeout(30000)


if __name__ == "__main__":
    asyncio.run(main())

Expected behavior

I expect web page to load

Actual behavior

Calling page.goto() throws NS_ERROR_PROXY_CONNECTION_REFUSED

Additional context

No response

Environment

- Operating System: MacOS 14.6.1
- CPU: Apple M3 Max
- Browser: Firefox
- Python Version: 3.12.5

[mxschmitt]

Instead of overriding the User-Agent like this, would our official API work for you?

        context = await browser.new_context(proxy=proxy, user_agent="Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:131.0) Gecko/20100101 Firefox/131.0")
        page = await context.new_page()