Can't start(or restart) Firefox in Private window. Can't pass args= to firefox.restart()

[tracywalker]

copied from iris2

Regarding test case https://testrail.stage.mozaws.net/index.php?/cases/view/120455 (private_browsing set)

Launch Firefox with a clean profile directly in Private window (add " -private" in the terminal at the end of the opening command)

@mwobensmith, how to start firefox test directly in private window?

previously we used construction restart_firefox(self, self.browser.path, self.profile_path, '', args=['-private'],...)

[pavelciapa]

#3111 and #3112 also needs this feature

[tracywalker]

Also related to issue #3207

[pavelciapa]

relates to #2703, fix of history_not_remembered

without this some test would fail, or should we use blocked by for these tests when we meet lack of this and other(#3207 , #3222 ) functionality?