https://docs.python.org/3/library/webbrowser.html#webbrowser.controller.open

browser_controller = webbrowser.get()

result = browser_controller.open(url)# <-- return False even though firefox successfully opens url

# expected behavior when url is opened successfully in browser it should return True

# like it return True for chrome and edge.

>>> import webbrowser
>>> brwsr = webbrowser.get("C:/Program Files/Mozilla Firefox/firefox.exe %s")
>>> brwsr.open('www.google.com')
False # <-- even though firefox opens the url

>>> brwsr = webbrowser.get("C:/Program Files (x86)/Google/Chrome/Application/chrome.exe %s")
>>> brwsr.open('www.google.com')
True
>>> brwsr = webbrowser.get(using ='windows-default')
>>> brwsr.open('www.google.com')
True
>>>

Fix: If you set firefox as your default browser then we get correct return from browser_controller.open(url)

Your code works fine on Windows 10 2004 with Python 3.8.3 and Firefox 77.0.1 (Firefox not set to default browser).

Your code is working fine on windows 10 64-bit and returning True as it opens firefox. python-3.9.2. The issue should be closed.

Others are reporting this works.  if that is not true on the most recent python 3.9 release, please reopen with more detailed reproduction instructions.