Reproducibility

Screenshot comparison only works if an unchanged theme renders identically twice. The throwaway profile pins what would otherwise drift: first-run tours, telemetry prompts, update checks and animations are off; pages are local files rather than live sites; and Nimbus/Normandy are disabled so Mozilla can't switch a toolbar feature on remotely between two runs.

Two CSS rules hide artifacts of the harness itself — the robot icon Firefox shows in automated sessions, and the rollout-gated IP Protection button. Neither is part of your theme.

Each session also picks its own Marionette port. Firefox's fixed default of 2828 means a browser leaked by an earlier run would silently accept the next session's connection, which shows up as your theme mysteriously not applying.