How can I set Firefox's viewport to a width of less than 450px? - selenium

Question

I'm trying to run some automated tests with Selenium in Firefox in normal view (not headless). I want to set the viewport to width=375, height=812 (iPhone X), but starting on Firefox 75 they set the minimum width to 450px (Firefox bug tracker).
I've read some posts saying you can use the userChrome.css file to modify this setting but they only seem to work on Linux and not on macOS nor Windows 10.
How can I force Firefox to allow me to set a viewport of less than 450px?

Answer 1

The solution is to create a profile.zip with the chrome/userChrome.css with the following content:
#main-window:not([chromehidden~="toolbar"]) {
min-width: 50px !important;
}
and a user.js with the following content:
user_pref("toolkit.legacyUserProfileCustomizations.stylesheets", true);
The profile.zip need to be given as a base64 encoded string to the remote web driver. Not sure how this is done in selenium.
PHP Web Driver Solution
Lot of spaghetti code but this is what worked for me using php web-driver library. Creating a profile and unpack the profile again and then add the userChrome.css. Tested on macOS:
// create firefox profile zip
$profileZipPath = __DIR__ . '/profile.zip';
$firefoxProfile = new FirefoxProfile();
$firefoxProfile->setPreference('toolkit.legacyUserProfileCustomizations.stylesheets', true);
$firefoxProfile = $firefoxProfile->encode();
file_put_contents($profileZipPath, base64_decode($firefoxProfile));
// unpack profile zp
$zip = new \ZipArchive();
$zip->open($profileZipPath);
$profilePath = __DIR__ . '/profile';
$zip->extractTo($profilePath);
// add chrome/userChrome.css
$cssDirectory = $profilePath . '/chrome';
mkdir($cssDirectory);
file_put_contents($cssDirectory . '/userChrome.css', <<<EOT
#main-window:not([chromehidden~="toolbar"]) {
min-width: 50px !important;
}
EOT
);
// create profile zip
$zip = new \ZipArchive();
$zip->open($profileZipPath, \ZipArchive::CREATE);
$dir = new \RecursiveDirectoryIterator($profilePath);
$files = new \RecursiveIteratorIterator($dir);
$dir_prefix = preg_replace(
'#\\\\#',
'\\\\\\\\',
$profilePath . DIRECTORY_SEPARATOR
);
foreach ($files as $name => $object) {
if (is_dir($name)) {
continue;
}
$path = preg_replace("#^{$dir_prefix}#", '', $name);
$zip->addFile($name, $path);
}
$zip->close();
// use the new profile
$firefoxProfile = base64_encode(file_get_contents($profileZipPath));
$capabilities = DesiredCapabilities::firefox();
$capabilities->setCapability(FirefoxDriver::PROFILE => $firefoxProfile);
// ...