Here's a step by step guide for making WebMs from porntube site sources.
1. In Firefox, use the Video DownloadHelper extension to download the video: https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/
2. Optional*: If the video you downloaded is incompatible with WebM for Retards, download Avidemux and learn to use it. With it you can cut and convert the video file into something you can use, preferably MP4: http://fixounet.free.fr/avidemux/
3: Open the video file in WebM for Retards (if file is incompatible, check step 2) and do what you want with it.
*Advanced techniques for Avidemux; In Avidemux you can convert convert the audio from stereo to mono, because WfR can only lower the bitrate, not make it mono. When it's mono the bitrate can be halved and the audio quality will be the same, it just won't be in stereo, but in WebMs where the audio is not the focus no-one gives a shit if it's mono. By doing this you halve the size of the audio part of the WebM, leaving more space for increased video quality, which people DO care about.
If the video is long you can also cut out the part you want in Avidemux instead of in WfR, since it's much easier to do in Avidemux than WfR.
If you have any questions, feel free to ask.