Recently tabs in titlebar are not tested on Linux which causes issues with various regressions there. We should enable testing of it on Ubuntu 18.04 as it's supported there.

Dao, do you know which and where the titlebar tests are placed? So far I found:

browser/tools/mozscreenshots/mozscreenshots/extension/configurations/TabsInTitlebar.jsm
toolkit/content/tests/chrome/chrome.ini
browser/components/customizableui/test/browser.ini

Thanks.