webdriverIO test on sauceLabs is not working for firefox - selenium-webdriver

Question

I am trying to run a wdio-test on saucelabs for chrome, IE and firefox.
test works fine for chrome and IE, however it fails for firefox with :
Infrastructure Error -- The Sauce VMs failed to start the browser or device.
I am using lastest version of wdio and sauce service:
"devDependencies": {
"#wdio/cli": "^6.1.5",
"#wdio/cucumber-framework": "^6.1.1",
"#wdio/local-runner": "^6.1.5",
"#wdio/sauce-service": "^6.1.0",
"#wdio/spec-reporter": "^6.1.5",
"#wdio/sync": "^6.1.5",
"chromedriver": "^81.0.0",
"wdio-chromedriver-service": "^6.0.2"
}
my browser config:
capabilities: [
{
maxInstances: 3,
browserName: "chrome",
browserVersion: "latest"
},
{
maxInstances: 3,
browserName: "firefox",
browserVersion: "latest",
platform: "windows 10",
"sauce:options": {
seleniumVersion: "3.14.0",
},
},
{
maxInstances: 3,
browserName: "internet explorer",
browserVersion: "latest"
},
],

Answer 1

This is an issue with how WebdriverIO and Sauce Labs handle W3C browser options. You do need to provide a sauce:options capability to use recent versions of Firefox, which would look like this:
capabilities: {
maxInstances: 3,
browserName: 'firefox',
platformName: 'Windows 10',
browserVersion: 'latest',
'sauce:options':
{'seleniumVersion': '3.14.0'}
}
The sauce:options specifies Sauce-only capabilities, such as which version of Selenium WebDriver to use in this case.

Answer 2

I was able to fix it.
In order for the W3C-compliant Selenium capabilities and protocol to work, all non-standard capabilities need to be defined inside the "sacue:options" block. This includes the "build" capability. Also, to specify a platform, the capability name has been changed from "platform" to "platformName". So the capabilities should look like this:
capabilities: {
browserName: 'firefox',
platformName: 'Windows 10',
browserVersion: 'latest',
'sauce:options':
{
'seleniumVersion': '3.14.0',
'build': buildName()
}
}