IP Address Change Notification (From - To) a must.
For a host of obvious reasons, its overdue, its about time, "get er done"!
Yes, there are Firefox Add-ons available for this, but they done work well, and require elevated Permissions, this should be a native feature/option browsers.
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You said it yourself that there are extensions doing that job already... How would making it built-in be making it any better? The diverse amount of add-ons is one of Firefox's strengths since day one (even as extensions became less powerful after Quantum); as much as possible I'd prefer a much leaner browser core that is easily extended by add-ons as necessary than having it bloated with many features not most people would need.