Bug Report: lots of build flake with Firefox via `-browsers`

**Describe the bug**

For the past month or so, we've been getting a lot more build flake in our CI builds when using Selenium with Firefox. You can see an example in [this build on an open source project](https://app.circleci.com/pipelines/github/mockdeep/questlog/5167/workflows/f9ee16c1-c314-4cb6-aa6c-fde3fd40efb9/jobs/23577). It complains about "Process unexpectedly closed with status 1" and every subsequent test fails. I enabled selenium logging for a bit more detail and got the following. In particular, the error "Error: cannot open display: :99" seems to suggest that the display is not available for some reason.

Selenium Logs

```
# Logfile created on 2025-04-20 12:24:54 -0700 by logger.rb/v1.7.0
2025-04-20 12:24:55 INFO Selenium [:logger_info] Details on how to use and modify Selenium logger:
https://selenium.dev/documentation/webdriver/troubleshooting/logging

2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Selenium Manager binary found at /home/circleci/app/vendor/bundle/ruby/3.4.0/gems/selenium-webdriver-4.31.0/bin/linux/selenium-manager
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Executing Process ["/home/circleci/app/vendor/bundle/ruby/3.4.0/gems/selenium-webdriver-4.31.0/bin/linux/selenium-manager", "--browser", "firefox", "--language-binding", "ruby", "--output", "json", "--debug"]
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Sending stats to Plausible: Props { browser: "firefox", browser_version: "", os: "linux", arch: "x86_64", lang: "ruby", selenium_version: "4.31" }
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] geckodriver not found in PATH
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] firefox detected at /usr/bin/firefox
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Running command: /usr/bin/firefox -v
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Output: "Mozilla Firefox 135.0"
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Detected browser: firefox 135.0
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Valid geckodriver versions for firefox 135: ["0.36.0", "0.35.0", "0.34.0"]
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Required driver: geckodriver 0.36.0
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] geckodriver 0.36.0 already in the cache
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Driver path: /home/circleci/.cache/selenium/geckodriver/linux64/0.36.0/geckodriver
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Browser path: /usr/bin/firefox
2025-04-20 12:24:55 DEBUG Selenium [:driver_service] Executing Process ["/home/circleci/.cache/selenium/geckodriver/linux64/0.36.0/geckodriver", "--port=4444"]
2025-04-20 12:24:55 DEBUG Selenium [:process] Starting process: ["/home/circleci/.cache/selenium/geckodriver/linux64/0.36.0/geckodriver", "--port=4444"] with {[:out, :err] => #, pgroup: true}
2025-04-20 12:24:55 DEBUG Selenium [:process] -> pid: 403
2025-04-20 12:24:55 DEBUG Selenium [:driver_service] polling for socket on ["127.0.0.1", 4444]
1745177095549 geckodriver INFO Listening on 127.0.0.1:4444
2025-04-20 12:24:55 DEBUG Selenium [:command] -> POST session
2025-04-20 12:24:55 DEBUG Selenium [:command] >>> http://127.0.0.1:4444/session | {"capabilities":{"alwaysMatch":{"acceptInsecureCerts":true,"browserName":"firefox","moz:firefoxOptions":{"binary":"/usr/bin/firefox","log":{"level":"trace"},"prefs":{"remote.active-protocols":3,"browser.download.folderList":2,"browser.download.manager.showWhenStarting":false,"browser.download.dir":"/home/circleci/app/tmp/downloads","browser.helperApps.neverAsk.saveToDisk":"text/csv"}},"moz:debuggerAddress":true}}}
2025-04-20 12:24:55 DEBUG Selenium [:header] > {"Accept" => "application/json", "Content-Type" => "application/json; charset=UTF-8", "User-Agent" => "selenium/4.31.0 (ruby linux)", "Content-Length" => "417"}
1745177095805 mozrunner::runner INFO Running command: MOZ_CRASHREPORTER="1" MOZ_CRASHREPORTER_NO_REPORT="1" MOZ_CRASHREPORTER_SHUTDOWN="1" "/usr/bin/firefox" "--m ... tte" "--remote-debugging-port" "9222" "--remote-allow-hosts" "localhost" "-no-remote" "-profile" "/tmp/rust_mozprofileDULmct"
1745177095806 geckodriver::marionette DEBUG Waiting 60s to connect to browser on 127.0.0.1
1745177095806 geckodriver::browser TRACE Failed to open /tmp/rust_mozprofileDULmct/MarionetteActivePort
1745177095806 geckodriver::marionette TRACE Retrying in 100ms
Error: cannot open display: :99
1745177095906 geckodriver::browser DEBUG Browser process stopped: exit status: 1
1745177095906 webdriver::server DEBUG <- 500 Internal Server Error {"value":{"error":"unknown error","message":"Process unexpectedly closed with status 1","stacktrace":""}}
2025-04-20 12:24:55 DEBUG Selenium [:header] <<< {"content-type" => ["application/json; charset=utf-8"], "cache-control" => ["no-cache"], "content-length" => ["105"], "date" => ["Sun, 20 Apr 2025 19:24:55 GMT"]}
2025-04-20 12:24:55 DEBUG Selenium [:command] <- {"value":{"error":"unknown error","message":"Process unexpectedly closed with status 1","stacktrace":""}}
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Executing Process ["/home/circleci/app/vendor/bundle/ruby/3.4.0/gems/selenium-webdriver-4.31.0/bin/linux/selenium-manager", "--browser", "firefox", "--browser-path", "/usr/bin/firefox", "--language-binding", "ruby", "--output", "json", "--debug"]
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] geckodriver not found in PATH
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Running command: /usr/bin/firefox -v
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Output: "Mozilla Firefox 135.0"
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Detected browser: firefox 135.0
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Required driver: geckodriver 0.36.0
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] geckodriver 0.36.0 already in the cache
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Driver path: /home/circleci/.cache/selenium/geckodriver/linux64/0.36.0/geckodriver
2025-04-20 12:24:55 DEBUG Selenium [:selenium_manager] Browser path: /usr/bin/firefox
2025-04-20 12:24:55 DEBUG Selenium [:driver_service] Executing Process ["/home/circleci/.cache/selenium/geckodriver/linux64/0.36.0/geckodriver", "--port=4445"]
2025-04-20 12:24:55 DEBUG Selenium [:process] Starting process: ["/home/circleci/.cache/selenium/geckodriver/linux64/0.36.0/geckodriver", "--port=4445"] with {[:out, :err] => #, pgroup: true}
2025-04-20 12:24:55 DEBUG Selenium [:process] -> pid: 445
2025-04-20 12:24:55 DEBUG Selenium [:driver_service] polling for socket on ["127.0.0.1", 4445]
1745177095956 geckodriver INFO Listening on 127.0.0.1:4445
2025-04-20 12:24:56 DEBUG Selenium [:command] -> POST session
2025-04-20 12:24:56 DEBUG Selenium [:command] >>> http://127.0.0.1:4445/session | {"capabilities":{"alwaysMatch":{"acceptInsecureCerts":true,"browserName":"firefox","moz:firefoxOptions":{"binary":"/usr/bin/firefox","log":{"level":"trace"},"prefs":{"remote.active-protocols":3,"browser.download.folderList":2,"browser.download.manager.showWhenStarting":false,"browser.download.dir":"/home/circleci/app/tmp/downloads","browser.helperApps.neverAsk.saveToDisk":"text/csv"}},"moz:debuggerAddress":true}}}
2025-04-20 12:24:56 DEBUG Selenium [:header] > {"Accept" => "application/json", "Content-Type" => "application/json; charset=UTF-8", "User-Agent" => "selenium/4.31.0 (ruby linux)", "Content-Length" => "417"}
1745177096206 webdriver::server DEBUG -> POST /session {"capabilities":{"alwaysMatch":{"acceptInsecureCerts":true,"browserName":"firefox","moz:firefoxOptions":{"bi ... d.dir":"/home/circleci/app/tmp/downloads","browser.helperApps.neverAsk.saveToDisk":"text/csv"}},"moz:debuggerAddress":true}}}
1745177096206 geckodriver::capabilities DEBUG Trying to read firefox version from ini files
1745177096207 geckodriver::capabilities DEBUG Found version 135.0
1745177096209 mozrunner::runner INFO Running command: MOZ_CRASHREPORTER="1" MOZ_CRASHREPORTER_NO_REPORT="1" MOZ_CRASHREPORTER_SHUTDOWN="1" "/usr/bin/firefox" "--m ... tte" "--remote-debugging-port" "9222" "--remote-allow-hosts" "localhost" "-no-remote" "-profile" "/tmp/rust_mozprofilegTVn1h"
1745177096209 geckodriver::marionette DEBUG Waiting 60s to connect to browser on 127.0.0.1
1745177096209 geckodriver::browser TRACE Failed to open /tmp/rust_mozprofilegTVn1h/MarionetteActivePort
1745177096209 geckodriver::marionette TRACE Retrying in 100ms
Error: cannot open display: :99
1745177096309 geckodriver::browser DEBUG Browser process stopped: exit status: 1
1745177096310 webdriver::server DEBUG <- 500 Internal Server Error {"value":{"error":"unknown error","message":"Process unexpectedly closed with status 1","stacktrace":""}}
2025-04-20 12:24:56 DEBUG Selenium [:header] <<< {"content-type" => ["application/json; charset=utf-8"], "cache-control" => ["no-cache"], "content-length" => ["105"], "date" => ["Sun, 20 Apr 2025 19:24:55 GMT"]}
2025-04-20 12:24:56 DEBUG Selenium [:command] <- {"value":{"error":"unknown error","message":"Process unexpectedly closed with status 1","stacktrace":""}}
2025-04-20 12:24:56 DEBUG Selenium [:selenium_manager] Executing Process ["/home/circleci/app/vendor/bundle/ruby/3.4.0/gems/selenium-webdriver-4.31.0/bin/linux/selenium-manager", "--browser", "firefox", "--browser-path", "/usr/bin/firefox", "--language-binding", "ruby", "--output", "json", "--debug"]
...
```

**To Reproduce**

This appears to happen with pretty much any Firefox/Selenium build on the Ruby image with `-browsers`, at least for me. You can check out [this PR](https://github.com/mockdeep/questlog/pull/2392) for a starting place.

**Expected behavior**

`geckodriver` should be able to reliably connect to the virtual display.

**Workarounds**

Right now we just end up re-running builds until they succeed, which is a pain.