geckodriver uses profiles to instrument Firefox’ behaviour. The user will usually rely on geckodriver to generate a temporary, throwaway profile. These profiles are deleted when the WebDriver session expires.
In cases where the user needs to use custom, prepared profiles, geckodriver will make modifications to the profile that ensures correct behaviour. See Automation preferences below on the precedence of user-defined preferences in this case.
Custom profiles can be provided two different ways:
--profile /some/locationto the
argscapability, which will instruct geckodriver to use the profile in-place;
or by setting the
profilecapability to a Base64-encoded ZIP of the profile directory.
Note that geckodriver has a known bug concerning
prevents the randomised Marionette port from being passed to
geckodriver. To circumvent this issue, make sure you specify the
port manually using
The second way is compatible with shipping Firefox profiles across
a network, when for example the geckodriver instance is running on
a remote system. This is the case when using Selenium’s
concept, where the WebDriver client and the server are running on
two distinct systems.
Default locations for temporary profiles¶
When a custom user profile is not provided with the
command-line argument geckodriver generates a temporary, throwaway
profile. This is written to the default system temporary folder
and subsequently removed when the WebDriver session expires.
The default location for temporary profiles depends on the system. On Unix systems it uses /tmp, and on Windows it uses the Windows directory.
The default location can be overridden. On Unix you set the
environment variable. On Windows, the following environment variables
are respected, in order:
It is not necessary to change the temporary directory system-wide. All you have to do is make sure it gets set for the environment of the geckodriver process:
% TMPDIR=/some/location ./geckodriver
As indicated in the introduction, geckodriver configures Firefox so it is well-behaved in automation environments. It uses a combination of preferences written to the profile prior to launching Firefox (1), and a set of recommended preferences set on startup (2).
These can be perused here:
As mentioned, these are recommended preferences, and any user-defined
preferences in the user.js file or as part of the
take precedence. This means for example that the user can tweak
browser.startup.page to override the recommended preference for
starting the browser with a blank page.
The recommended preferences set at runtime (see 2 above) may also
be disabled entirely by setting
remote.prefs.recommended starting with Firefox
91. For older versions of Firefox, the preference to use was
This may however cause geckodriver to not behave correctly according
to the WebDriver standard, so it should be used with caution.
Users should take note that the
marionette.port preference is
special, and will always be overridden when using geckodriver unless
--marionette-port <port> flag is used specifically to instruct
the Marionette server in Firefox which port to use.