Use Firefox for Android or Firefox OS and navigate to a help topic that includes subtopics (related topics). Then choose one of those subtopics from the menu and you get the 404 page.
See bug 867701 comment 29 and above
Status: NEW → RESOLVED
Last Resolved: 6 years ago
Resolution: --- → DUPLICATE
Duplicate of bug: 867701
Apparently it happens without the flag too. But the new pages should resolve this.
Status: RESOLVED → REOPENED
Resolution: DUPLICATE → ---
This should be fixed now with the fix to bug 867701
Status: REOPENED → RESOLVED
Last Resolved: 6 years ago → 6 years ago
Resolution: --- → FIXED
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