Selenium "Unable to find a matching set of capabilities" despite driver being in /usr/local/bin

I'm trying to follow a tutorial about Selenium, http://selenium-python.readthedocs.io/getting-started.html. I've downloaded the latest version of geckodriver and copied it to /usr/local/bin. However, when I try

from selenium import webdriver
driver = webdriver.Firefox()

I get the following error message:

Traceback (most recent call last):
  File "/Users/kurtpeek/Documents/Scratch/selenium_getting_started.py", line 4, in <module>
    driver = webdriver.Firefox()
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium/webdriver/firefox/webdriver.py", line 152, in __init__
    keep_alive=True)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 98, in __init__
    self.start_session(desired_capabilities, browser_profile)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 188, in start_session
    response = self.execute(Command.NEW_SESSION, parameters)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 252, in execute
    self.error_handler.check_response(response)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium/webdriver/remote/errorhandler.py", line 194, in check_response
    raise exception_class(message, screen, stacktrace)
selenium.common.exceptions.WebDriverException: Message: Unable to find a matching set of capabilities

[Finished in 1.2s with exit code 1]

From https://github.com/SeleniumHQ/selenium/issues/3884, it seems like other users are experiencing similar issues, but the Selenium team is unable to reproduce it. How can I get Selenium working with Firefox? (It does work with chromedriver and a webdriver.Chrome() instance, so I suspect this might be a bug in Selenium).

Updating Firefox and Selenium solved it for me. I don't pretend to have an explanation for the root cause however.

Updated Firefox 48 → 53
Updated to Selenium 3.4.1

I also reinstalled/updated Geckodriver using Homebrew and explicitly used it as an executable for Selenium WebDriver, but it turned out that it wasn't necessary to mitigate the "Unable to find matching set of capabilities" error.

I had this same issue, and the problem was related to using Firefox ESR (I'm on Debian). To be more specific, I'm on Debian 10 using 64-bit Firefox 68.11.0esr, python3.7, selenium 3.141.0, and geckodriver 0.27.0.

Here's the standard example I used that failed:

from selenium import webdriver

browser = webdriver.Firefox()
browser.get("http://google.com")

As recommended in this answer, I changed:

browser = webdriver.Firefox()

browser = webdriver.Firefox(firefox_binary="/usr/bin/firefox-esr")

and it worked.

If you don't know the path to firefox-esr, you can run sudo find / -name firefox-esr on the command line. Several should come up.

Selenium "Unable to find a matching set of capabilities" despite driver being in /usr/local/bin

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