There is no footage of the beginning of the fire. As noted in the Wikipedia page about the disaster, "Although the landing was being filmed by cameramen from four newsreel teams and at least one spectator, with numerous photographers also being at the scene, no footage or photographs are known to exist of the moment the fire started."

All available videos contain footage from one of two newsreel cameras. One captures the fire slightly ahead of the other.

In the earlier one, the camera pans quickly upward. In this video that occurs at 3:31
https://www.youtube.com/watch?v=ZfVsCU59GKM

The other shows the airship already sinking down at a steep angle. The Pathe newsreel in which this occurs at 2:50 contains bizarre elements.
https://www.youtube.com/watch?v=CgWHbpMVQ1U

Most significantly, within the white flash that leads into the clip of the fire, at least four single frames, below, are present containing unrelated imagery.

A colorized video of footage from what is apparently a third newsreel camera has recently been uploaded:
https://www.youtube.com/watch?v=_TUjUhuF5PQ
Starting at 0:21 it would appear that the cameraman was well positioned to capture the beginning of the fire, yet there is a break at 0:24 that suggests that he was out to lunch during the crucial opening seconds of the fire, leaving a significant gap -- perhaps at least 10 seconds -- in the timeline.
Incidentally, the following sequence of frames from that video exposes some of the decision-making about what to colorize when there's a hole in the film:

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