How to opt out of 32-bit Firefox migration to 64-bit
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Users of 32-bit Firefox on 64-bit Windows systems with at least 3 GB RAM are being migrated to 64-bit Firefox, for increased stability and security. See this Mozilla blog post and the Firefox/Win64 Mozilla Wiki for more information.
This migration began on October 9, 2017, with the release of Firefox version 56.0.1. When you update 32-bit Firefox from version 56.0 to a newer version, an automatic migration to 64-bit Firefox takes place during the update process.
If you prefer to use 32-bit Firefox, you can go to this Firefox download page, download the latest Windows 32-bit installer for your language and reinstall Firefox.
Mozilla will not migrate users with 32-bit Firefox version 56.0.1 or later.