Via PCWorld:
See the referenced article for additional information.
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The Mozilla Foundation will soon no longer support users of Windows 7, 8, 8.1 and macOS 10.12, 10.13 and 10.14 with updates for the popular Firefox browser. For a short time, an ESR (Extended Support Release) or LTS (Long-term Support) version will be available for these users before support finally ends in 2024. Mozilla announced the information in its own support blog.
See the referenced article for additional information.