Firefox not reachable in several jobs

09:14:51 [INFO] Running org.nuxeo.functionaltests.ITLoginLogoutTest
09:14:51 [ERROR] Tests run: 1, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 0.011 s <<< FAILURE! - in org.nuxeo.functionaltests.ITLoginLogoutTest
09:14:51 [ERROR] org.nuxeo.functionaltests.ITLoginLogoutTest  Time elapsed: 0.01 s  <<< ERROR!
09:14:51 org.openqa.selenium.WebDriverException: 
09:14:51 Failed to connect to binary FirefoxBinary(/usr/bin/firefox) on port 7055; process output follows: 
09:14:51 Error: no display specified
09:14:51 
09:14:51 Build info: version: '2.53.0', revision: '35ae25b', time: '2016-03-15 17:00:58'
09:14:51 System info: host: 'd7b4c63612d9', ip: '172.17.0.32', os.name: 'Linux', os.arch: 'amd64', os.version: '4.4.0-104-generic', java.version: '1.8.0_191'
09:14:51 Driver info: driver.version: FirefoxDriver
09:14:51 	at org.openqa.selenium.firefox.internal.NewProfileExtensionConnection.start(NewProfileExtensionConnection.java:124)
09:14:51 	at org.openqa.selenium.firefox.FirefoxDriver.startClient(FirefoxDriver.java:271)
09:14:51 	at org.openqa.selenium.remote.RemoteWebDriver.<init>(RemoteWebDriver.java:119)
09:14:51 	at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:218)
09:14:51 	at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:211)
09:14:51 	at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:129)
09:14:51 	at