incomplete waterfall with firefox

[nilskuhn]

I've tested with different versions of the agent-(wptdriver-)components:
* agent-/wptdriver-version 77 (WPT 2.8)
* agent-/wptdriver-version 87 (WPT 2.9)
* agent-/wptdriver-version 96
* actual agent-/wptdriver-version 116

For the older versions 77 and 87 I had to downgrade FF to version 17 because with version 21 I just got blank page and tests didn't start.
I've incomplete waterfalls in all versions with the given script. The result-fie 1_IEWTR.txt on server already contains too less requets, so the measurement itself seems to start too late (after setting logData 1). It's definitely no problem with the newest version.