Facebook currently has no speed playback controls for their videos to speed or slow them down, much like how YouTube does it.
Until Facebook itself implements the feature, may this Chrome extension serve as your interim solution.
The Chrome extension is available now from the Chrome Web Store
git clone https://github.com/RemLampa/facebook-video-speed-control.git
cd facebook-video-speed-control
yarn install
yarn upgrade
npm run dist
Then you can visit chrome://extensions/ in Chrome and enable "Developer mode" and then click "Load unpacked" and choose the facebook-video-speed-control\dist folder.
I will update this document to detail how interested people can contribute to the code. But everyone is free to file feature requests and bug reports on this repo's Issues page.
Refer to the license file.