Download

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.

Concatenate

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.}

concat demuxer

Make a text file listing your inputs:

file 'input0.webm'
file 'input1.webm'
file 'input2.webm'

Now run ffmpeg:

ffmpeg -f concat -i input.txt -c copy output.webm