* Bugs that have too little information to be implemented, and just make everyone who stares at them decide to move onto another bug. These do get cleaned up during periodic deep triages, so I don't have a good 4-digit bug for this.
All citations are rare 4-digit bugs or super-rare 3-digit bugs on Mozilla's bugzilla instance. I myself have triaged a few of those out of existence, but I never fixed any of them.
In my experience, doing bug triage, old bugs generally fall into a few categories:
* Feature requests that aren't WONTFIX but so low priority that they'll stay on the bottom of the list forever (e.g., https://bugzilla.mozilla.org/show_bug.cgi?id=2892).
* Bugs that are impossible to fix in the current architecture, and are too low priority to justify the architectural changes to fix them (e.g., https://bugzilla.mozilla.org/show_bug.cgi?id=9942).
* Bugs that have too little information to be implemented, and just make everyone who stares at them decide to move onto another bug. These do get cleaned up during periodic deep triages, so I don't have a good 4-digit bug for this.
* Bugs that are political. (https://bugzilla.mozilla.org/show_bug.cgi?id=540)
* Bugs that exist in components that never get triage because no one works on that stuff (or the project itself may be dead) (https://bugzilla.mozilla.org/show_bug.cgi?id=1334).
All citations are rare 4-digit bugs or super-rare 3-digit bugs on Mozilla's bugzilla instance. I myself have triaged a few of those out of existence, but I never fixed any of them.