Encoding Movies into webm format

Get FFmepg with webm support:
you can get the lastest nightly builds of ffmpeg fromfirefogg site:

download the executable for your platform.
(linux, windows, mac are supported)

Start encoding:
simple command is like:
ffmpeg -i MVI_6646.AVI -b 600k -aq 60 MVI_6646.webm

Bit more Advanced
ffmpeg -y -i MOV003.3gp -threads 0 -f webm -vcodec libvpx -g 250 -qmin 10 -qmax 42 -rc_buf_aggressivity 1 -vb 800k -acodec libvorbis -aq 0 MOV003.webm

Use VP9 and Opus for encoding
ffmpeg -i 00019.MTS -c:v libvpx-vp9 -crf 10 -threads 1 -speed 0 -tile-columns 0 -frame-parallel 0 -auto-alt-ref 1 -lag-in-frames 25 -g 9999 -aq-mode 0 -c:a libopus -b:a 64k -f webm out.web
Webm parameters
Webm to ffmpeg parameter mapping