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This page will help Commons users with firefogg, a firefox addon which supports video encoding to OGV and WebM. Also UploadWizard supports Firefogg and enables automatic upload of almost any video file regardless of source codec!
Using Firefogg for saving videos locally 
Visit firefogg.org/make select your video file then save it to disk.
In addition to encoding the file, firefogg allows other options to be set, including removing audio and changing the aspect ratio. Quality setting should be set on highest quality (ie. '10' in the advanced settings or '6Mbit connection' in the simple settings). All converting is done on your computer; the file is never uploaded on the firefogg website.
Supported file types 
Firefogg builds on the ffmpeg library, which supports a wide range of codecs and containers. In general it should support most files you have.