Firefox versions up to v115 ESR (Extended Support Release) maintain compatibility with Windows 8.1. You can download the portable version from trusted repositories like PortableApps.
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Note: DuckDuckGo for Windows requires Windows 10 (May 2020) or later. DuckDuckGo Download the DuckDuckGo Browser for Windows Firefox versions up to v115 ESR (Extended Support
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Spyware that defeats the very privacy purpose you sought in the first place.