The first step is to download
ffmpeg. It's a standalone binary so you don't need to install it. Just execute it directly.
Your (expired) Ubuntu version offers
avconv which is missing many features including several concatenation functions.
Use the concat demuxer if you want to attempt to join them with no re-encoding. All videos must have the same parameters.
Use the concat filter if the videos vary in width, height, frame rate, etc. The filter will require re-encoding.
Note: You never did provide the info I requested a year and a half ago in my comments to your question, so I can't suggest which one you specifically need to use. Also, I can't provide examples specific to your inputs without this info, so the following examples are generic and may not work without additional options.
Make a text file listing your inputs:
ffmpeg -f concat -i input.txt -c copy output.webm
In this example
input1.webm has a bigger width x height than the others. This example command will scale
input1.webm so it matches the other videos:
ffmpeg -i input0.webm -i input1.webm -i input2.webm -filter_complex \
-map "[v]" -map "[a]" output.webm