[firefox]: for some tests HUD never shows up

[legomushroom]

For some tests, HUD never shows up:

Repro:

1. Launch express.test.app
2. Go to Timeline, click on Http Services

Consistency: Firefox only

[mike-kaufman]

need to verify if this is an issue with HUD or an issue w/ our tests.

[davidkpiano]

The issue is two-fold:

On Firefox:

an error to endpoint /timeline/http-services/streamResponse failed with error TypeError: NetworkError when attempting to fetch resource.

And there is an issue where, because the HUD didn't exist yet (UI blocking or something), DOM operations were attempted on nodes that did not exist yet, so it was throwing errors. (PR for this soon)

[philliphoff]

With the latest HUD bits I'm still seeing this issue (on Firefox only, only for the Timeline::Http Services tests).

[philliphoff]

Still seeing this issue on Firefox on v0.22.7. However, it seems to be restricted to only the test "suites" that are fetch()-based. When running the XHR-based equivalent suites, the HUD is shown immediately. Perhaps there's a slightly different update/refresh path between the HUD's fetch() and XHR proxies?