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+🟩 Previous: [Apply + Update + Maintain](https://github.com/arkenfox/user.js/wiki/3.4-Apply---Update--Maintain)
+
+🟥 Summary: **ALWAYS run prefsCleaner after updating**
+
+---
+
+🟪 The Issue
+
+Over time, Firefox deprecates prefs, and arkenfox also removes prefs or makes them inactive (commented out). It does this because they are no longer needed or may cause issues
+
+For the duration of each ESR life cycle
+ - `0650s` - we keep a list of prefs removed by arkenfox
+ - `9999s` - we keep a list of prefs deprecated by firefox
+
+A master copy of everything that has ever been in the arkenfox user.js but is no longer, is kept up to date in our [cleanup script](https://github.com/arkenfox/user.js/blob/master/scratchpad-scripts/arkenfox-cleanup.js)
+
+So these removed, deprecated and inactive prefs should be reset - to avoid issues and keep things tidy. Doing it manually is time consuming and prone to human error
+
+🟪 Solution
+
+The solution is simple. Script it, and reset **_every single pref_** in the user.js regardless of whether it is active or inactive. Then restart Firefox and all your ACTIVE prefs are reapplied. The end result being all your INACTIVE ones have been reset.
+
+And that's exactly what the `prefsCleaner` script does
+
+So ALWAYS run prefsCleaner, with Firefox closed, after each update
+
+---
+
+🟩 Next: [Extensions](https://github.com/arkenfox/user.js/wiki/4.1-Extensions)
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