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In order to improve performance and focus on technologies that use Web standards, Firefox has dropped support for NPAPI plugins except for Flash, starting with version 52 (see this article for details).
If you use Google Hangouts, you will have issues with audio and video calls, but chat will still work. This is a temporary issue while Google develops a way to make Hangouts work without plugins. See the official Google communication on this issue.
In the meantime, you can download and install the Extended Support Release (ESR) version of Firefox 52 until Google releases a fix. Important: Choose the Windows (32-bit) download.