The webbrowser opens almost all browsers in background mode, it looks like only Elinks is not opened in this mode.

Problem: the webbrowser doesn't read the exit status of the browser, and so create zombi process. On Python 3.6, the subprocess module now emits a ResourceWarning in this case (issue #26741).

For example, the following script shows a zombi process ("<defunc>"):
---
import webbrowser, os

b = webbrowser.get("firefox")
b.open("https://bugs.python.org/issue26741")
b = None

os.system("ps ax|grep firefox")
input("press ENTER when firefox exited")
os.system("ps ax|grep firefox")
---

I guess that Python should use os.fork() somehow to fix this issue.

Another option is to hack the Popen object to not display the warning, but I don't think that it's a safe option.

See also the issue #27068 "Add a detach() method to subprocess.Popen", but I don't think that it is directly related to background processes run by webbrowser.

I checked how "xdg-open http://www.python.org/" works. It looks like the command works as webbrowser: create a child process "firefox URL", but then it waits until the command completes.

In fact, "firefox URL" exits quickly. So we can try "background = False" for firefox. Well, at least with my Firefox 46.0.1.

I think the Firefox command will exit immediately if it just signals an existing Firefox process and window, but becomes the main Firefox process if there isn’t already one running.

Background = False sounds like it won’t help. Don’t we want the opposite?

This appears to be Firefox specific? At least:

b = webbrowser.get("chromium-browser")
b.open("https://bugs.python.org/issue26741")

returns simply:

True

With no exception

Isn't this a duplicate of issue5993?

It does look similar. They probably could be merged. The main difference is in Issue 5993 Eivind suggested to somehow use a “wait” system call, while here Victor suggested “fork” (perhaps to orphan a grandchild process?).