Transcoding to webm:
$ ffmpeg -i video.mpg -s 1280x760 -vpre libvpx-720p -b 3900k -pass 1 -an -f webm -y video.webm

Output:
ffmpeg version 0.10.11-7:0.10.11-1~precise1 Copyright (c) 2000-2014 the FFmpeg developers
  built on Feb  6 2014 16:53:05 with gcc 4.6.3
  configuration: --arch=amd64 --disable-stripping --enable-pthreads --enable-runtime-cpudetect --extra-version='7:0.10.11-1~precise1' --libdir=/usr/lib/x86_64-linux-gnu --prefix=/usr --enable-bzlib --enable-libdc1394 --enable-libfreetype --enable-frei0r --enable-gnutls --enable-libgsm --enable-libmp3lame --enable-librtmp --enable-libopencv --enable-libopenjpeg --enable-libpulse --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-vaapi --enable-vdpau --enable-libvorbis --enable-libvpx --enable-zlib --enable-gpl --enable-postproc --enable-libcdio --enable-x11grab --enable-libx264 --shlibdir=/usr/lib/x86_64-linux-gnu --enable-shared --disable-static
  libavutil      51. 35.100 / 51. 35.100
  libavcodec     53. 61.100 / 53. 61.100
  libavformat    53. 32.100 / 53. 32.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 61.100 /  2. 61.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  6.100 /  0.  6.100
  libpostproc    52.  0.100 / 52.  0.100
[mpeg @ 0x1242680] max_analyze_duration 5000000 reached at 5000000
Input #0, mpeg, from 'dubna-cookies.mpg':
  Duration: 00:01:09.80, start: 1.000000, bitrate: 4105 kb/s
    Stream #0:0[0x1e0]: Video: mpeg1video, yuv420p, 1280x760 [SAR 1:1 DAR 32:19], 104857 kb/s, 25 fps, 25 tbr, 90k tbn, 25 tbc
    Stream #0:1[0x1c0]: Audio: mp2, 48000 Hz, stereo, s16, 128 kb/s
Please use -b:a or -b:v, -b is ambiguous
[buffer @ 0x125a5a0] w:1280 h:760 pixfmt:yuv420p tb:1/1000000 sar:1/1 sws_param:
[scale @ 0x125a100] w:1280 h:760 fmt:yuv420p -> w:1280 h:720 fmt:yuv420p flags:0x4
[libvpx @ 0x12dad20] v1.0.0
Output #0, webm, to 'dubna-cookies.webmc':
  Metadata:
    encoder         : Lavf53.32.100
    Stream #0:0: Video: vp8, yuv420p, 1280x720 [SAR 18:19 DAR 32:19], q=11-51, pass 1, 3900 kb/s, 1k tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (mpeg1video -> libvpx)
Press [q] to stop, [?] for help
frame= 1747 fps= 76 q=0.0 Lsize=       0kB time=00:00:00.00 bitrate=   0.0kbits/s
video:0kB audio:0kB global headers:0kB muxing overhead inf%
Output file is empty, nothing was encoded (check -ss / -t / -frames parameters if used)