Best to keep Sync turned off, especially if you store passwords within Firefox.
Instead, turn on auto-fill in the Android settings for your default password manager app.
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Sync data passphrase is only required when installing on a new device. Once loaded, the data will be there.
If your device is stolen and phone happens to be unlocked, they can view your past history, extensions, bookmarks etc.
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Other mainstream browsers:
- DuckDuckGo Privacy browser
- Brave browser
- Focus by Firefox
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Android 9 or higher:
- Set up Private DNS with a privacy DNS — good for adblocking
- Or use VPN with a privacy DNS
You do need to go overboard, but if you want to isolate your desktop from your mobile, use different browsers.