So far, we have learned about Firefox profiles, and how we can create our own customized profiles for Firefox Driver. Now, let's see how we can set our preferences in the profiles we create and where FirefoxDriver stores them.
According to Mozilla, a Firefox Preference is any value or defined behavior that can be set by a user. These values are saved to the preference files. If you open the profile directory by navigating to Help | Troubleshooting Information and clicking on the Show Folder button, you will see two preference files: prefs.js and user.js. All the user preferences are written to the prefs.js file by the Firefox application during the launch. A user can override those values for their chosen values, and they are stored in the user.js file. The value in user.js for a preference takes precedence over all the other values set for that particular preference. So, your FirefoxDriver overwrites all the default preferences of Firefox in the user.js file for you. When you add a new preference, FirefoxDriver writes that to the user.js preference file, and the Firefox browser behaves accordingly.
Open the user.js file in the profile directory. The following are the list of all the preferences that FirefoxDriver sets for you by default:
user_pref("app.normandy.api_url", "");
user_pref("app.update.auto", false);
user_pref("app.update.enabled", false);
user_pref("browser.EULA.3.accepted", true);
user_pref("browser.EULA.override", true);
user_pref("browser.displayedE10SNotice", 4);
user_pref("browser.dom.window.dump.enabled", true);
user_pref("browser.download.manager.showWhenStarting", false);
user_pref("browser.laterrun.enabled", false);
user_pref("browser.link.open_external", 2);
user_pref("browser.link.open_newwindow", 2);
user_pref("browser.newtab.url", "about:blank");
user_pref("browser.newtabpage.enabled", false);
user_pref("browser.offline", false);
user_pref("browser.reader.detectedFirstArticle", true);
user_pref("browser.safebrowsing.blockedURIs.enabled", false);
user_pref("browser.safebrowsing.downloads.enabled", false);
user_pref("browser.safebrowsing.enabled", false);
user_pref("browser.safebrowsing.malware.enabled", false);
user_pref("browser.safebrowsing.passwords.enabled", false);
user_pref("browser.safebrowsing.phishing.enabled", false);
user_pref("browser.search.update", false);
user_pref("browser.selfsupport.url", "");
user_pref("browser.sessionstore.resume_from_crash", false);
user_pref("browser.shell.checkDefaultBrowser", false);
user_pref("browser.showQuitWarning", false);
user_pref("browser.snippets.enabled", false);
user_pref("browser.snippets.firstrunHomepage.enabled", false);
user_pref("browser.snippets.syncPromo.enabled", false);
user_pref("browser.startup.homepage", "about:blank");
user_pref("browser.startup.homepage_override.mstone", "ignore");
user_pref("browser.startup.page", 0);
user_pref("browser.tabs.closeWindowWithLastTab", false);
user_pref("browser.tabs.warnOnClose", false);
user_pref("browser.tabs.warnOnOpen", false);
user_pref("browser.uitour.enabled", false);
user_pref("browser.usedOnWindows10.introURL", "about:blank");
user_pref("browser.warnOnQuit", false);
user_pref("datareporting.healthreport.about.reportUrl", "http://%(server)s/dummy/abouthealthreport/");
user_pref("datareporting.healthreport.documentServerURI", "http://%(server)s/dummy/healthreport/");
user_pref("datareporting.healthreport.logging.consoleEnabled", false);
user_pref("datareporting.healthreport.service.enabled", false);
user_pref("datareporting.healthreport.service.firstRun", false);
user_pref("datareporting.healthreport.uploadEnabled", false);
user_pref("datareporting.policy.dataSubmissionEnabled", false);
user_pref("datareporting.policy.dataSubmissionPolicyAccepted", false);
user_pref("datareporting.policy.dataSubmissionPolicyBypassNotification", true);
user_pref("devtools.errorconsole.enabled", true);
user_pref("dom.disable_open_during_load", false);
user_pref("dom.ipc.reportProcessHangs", false);
user_pref("dom.max_chrome_script_run_time", 30);
user_pref("dom.max_script_run_time", 30);
user_pref("dom.report_all_js_exceptions", true);
user_pref("extensions.autoDisableScopes", 10);
user_pref("extensions.blocklist.enabled", false);
user_pref("extensions.checkCompatibility.nightly", false);
user_pref("extensions.enabledScopes", 5);
user_pref("extensions.installDistroAddons", false);
user_pref("extensions.logging.enabled", true);
user_pref("extensions.shield-recipe-client.api_url", "");
user_pref("extensions.showMismatchUI", false);
user_pref("extensions.update.enabled", false);
user_pref("extensions.update.notifyUser", false);
user_pref("focusmanager.testmode", true);
user_pref("general.useragent.updates.enabled", false);
user_pref("geo.provider.testing", true);
user_pref("geo.wifi.scan", false);
user_pref("hangmonitor.timeout", 0);
user_pref("javascript.enabled", true);
user_pref("javascript.options.showInConsole", true);
user_pref("marionette.log.level", "INFO");
user_pref("marionette.port", 51549);
user_pref("network.captive-portal-service.enabled", false);
user_pref("network.http.phishy-userpass-length", 255);
user_pref("network.manage-offline-status", false);
user_pref("network.sntp.pools", "%(server)s");
user_pref("offline-apps.allow_by_default", true);
user_pref("plugin.state.flash", 0);
user_pref("prompts.tab_modal.enabled", false);
user_pref("security.csp.enable", false);
user_pref("security.fileuri.origin_policy", 3);
user_pref("security.fileuri.strict_origin_policy", false);
user_pref("services.settings.server", "http://%(server)s/dummy/blocklist/");
user_pref("signon.rememberSignons", false);
user_pref("startup.homepage_welcome_url", "");
user_pref("startup.homepage_welcome_url.additional", "about:blank");
user_pref("toolkit.networkmanager.disable", true);
user_pref("toolkit.startup.max_resumed_crashes", -1);
user_pref("toolkit.telemetry.enabled", false);
user_pref("toolkit.telemetry.prompted", 2);
user_pref("toolkit.telemetry.rejected", true);
user_pref("toolkit.telemetry.server", "https://%(server)s/dummy/telemetry/");
user_pref("webdriver_accept_untrusted_certs", true);
user_pref("webdriver_assume_untrusted_issuer", true);
user_pref("xpinstall.signatures.required", false);
user_pref("xpinstall.whitelist.required", false);
This Firefox Driver treats them as Frozen Preferences and doesn't allow the test-script developer to change them. However, there are a few preferences in the preceding list that FirefoxDriver allows you to change, which we will see shortly.