Test Desktop Targeting (Beta Firefox) taking longer than expected

[jaredlockhart]

Looking at our desktop targeting integration tests, the run times look like:

Test Desktop Targeting (Release Firefox)
145 passed, 1 skipped in 202.46s (0:03:22)

Test Desktop Targeting (Nightly Firefox)
145 passed, 1 skipped in 205.31s (0:03:25)

Test Desktop Targeting (Beta Firefox)
145 passed, 1 skipped in 625.02s (0:10:25)

Seems like something must be slowing the beta tests down, and since this is now the single slowest integration run, improving this would improve our overall PR throughput.

Let's see if we can figure out and fix whatever's slowing the beta targeting tests down.

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