testcafe: Parallel Firefox does not work using TestCafe Docker Image

What is your Test Scenario?

Parallel Firefox does not work using TestCafe Docker Image. Tried many times with many different settings and it only works when concurrency is set to 1. 2x fixture files that go to https://www.google.com and check visibility of logo

Tried:

# Tried
docker run --rm -it -v tests/:/tests --shm-size=4g testcafe:latest '-c 2 firefox:headless' tests/**/*

# Tried
docker run --rm -it -v tests/:/tests --shm-size=4g testcafe:latest '-c 2 firefox -headless --incognito --disable-dev-shm-usage --display=:1.0 --profile="./firefox-profile" -MOZ_LOG_FILE="./asd.txt" -MOZ_LOG="timestamp,rotate:200,nsHttp:5,cache2:5,nsSocketTransport:5,nsHostResolver:5,sync"' tests/**/*

# Tried
docker run -v /app/:/app -v /tmp/.X11-unix:/tmp/.X11-unix --shm-size=4g -it --entrypoint /bin/sh testcafe/testcafe:latest
dbus-daemon --session --fork
Xvfb :1 -screen 0 "1280x720x24" >/dev/null 2>&1 &
export DISPLAY=:1.0
fluxbox >/dev/null 2>&1 &
PARALLEL=2 node ./node_modules/.bin/ts-node ./run-tests.ts

What is the Current behavior?

TestCafe runs indefinitely - does not end - or it fails with following error:

{ Error: Unable to establish one or more of the specified browser connections. This can be caused by network issues or remote device failure.
    at BrowserSet._waitConnectionsOpened (/app/e2e-tests/node_modules/testcafe/lib/runner/browser-set.js:64:30)
    at pinkie_1.default.resolve.then (/app/e2e-tests/node_modules/testcafe/lib/runner/browser-set.js:81:31)
    at invokeCallback (/app/e2e-tests/node_modules/pinkie/index.js:60:12)
    at Array.forEach (<anonymous>:null:null)
    at publish (/app/e2e-tests/node_modules/pinkie/index.js:147:32)
    at Array.publishFulfillment (/app/e2e-tests/node_modules/pinkie/index.js:152:2)
    at Immediate.asyncFlush (/app/e2e-tests/node_modules/pinkie/index.js:17:19)
    at runCallback (timers.js:706:11)
    at tryOnImmediate (timers.js:676:5)
    at processImmediate (timers.js:658:5)
    at process.topLevelDomainCallback (domain.js:120:23)
 code: 'E1004', data: [] }

What is the Expected behavior?

Firefox works properly in parallel using provided TestCafe Docker image.

Steps to Reproduce:

1. Create 2 simple specs checking google.com
2. Execute TestCafe using docker image with “firefox:headless”
3. Check if TestCafe passes

Your Environment details:

• testcafe version: 1.5.0 + 1.4.0 + 1.2.1
• node.js version: 10.15
• command-line arguments: firefox:headless
• browser name and version: Firefox 68.0.0 / Linux 0.0.0
• platform and version: Docker TestCafe started on macOS 10.14.5