This is not a Fission-specific bug (only 22% of these crash reports have Fission enabled), though Fission does seem to exacerbate this crash signature. Less than 4% of other Nightly crashes have Fission enabled.
Bug 1603976 Comment 5 Edit History
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~This is not a Fission-specific bug (only 22% of these crash reports have Fission enabled), though Fission does seem to exacerbate this crash signature. Less than 4% of other Nightly crashes have Fission enabled.~ Correction: This crash is Fission-specific. Andrew points out (in comment 9 below) that the crash reports with this signature and reason "Attempt to deserialize absent BrowsingContext" are all Fission.