Here is the download log, I thought that I had uploaded it. My mistake.

Entry date: 22/12/2013 20:42:30 FFmpeg version UNKNOWN, Copyright (c) 2000-2008 Fabrice Bellard, et al.

 configuration: --enable-shared --disable-static --enable-memalign-hack --extra-cflags=-fno-common --enable-gpl --enable-postproc --enable-swscale --enable-avfilter --enable-pthreads --enable-avisynth --enable-libfaac --enable-libfaad --enable-libgsm --enable-libmp3lame --enable-libnut --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-libvorbis --enable-libx264 --enable-libxvid --enable-zlib
 libavutil     49.12. 0 / 49.12. 0
 libavcodec    52. 7. 0 / 52. 7. 0
 libavformat   52.23. 1 / 52.23. 1
 libavdevice   52. 1. 0 / 52. 1. 0
 libavfilter    0. 1. 0 /  0. 1. 0
 libswscale     0. 6. 1 /  0. 6. 1
 libpostproc   51. 2. 0 / 51. 2. 0
 built on Dec 21 2008 21:26:46, gcc: 3.4.5 (mingw-vista special r3)

C:\Users\CJ\AppData\Local\Temp\????? ?????? ?? ???? ???.3gp: no such file or directory Conversion failed

And here is the Youtube page from which it came (not that this happens with all video downloads I try to convert): http://www.youtube.com/watch?v=USjvfD0XSmI

Firefox Download Helper ??
Are under the assumption that add-on is made by Mozilla?

If this - https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/ - is what you installed, that is a 3rd party add-on for Firefox, as are 99.9% of the extensions hosted at Addons.Mozilla.org .

This - http://www.downloadhelper.net/documentation.php - is listed on the download page for that extension as the Support Site.

Just as I feared. No, I am aware this was not made by Mozilla. But their support page (which you linked to) leads to a forum which comes up as an unavailable page! I fear that I have been duped! I ask here only because I do not know where else to go. Is there anyone with Downloadhelper installed who can help?

I edited the title of this thread to attract other users of that extension, as I don't use that extension or any other download helpers and can't help you myself.

IMO, the biggest problem with Video DownloadHelper and other similar add-ons is that their "target" is constantly moving. Downloading (instead of playing them in a web browser as intended) most videos on the internet violates the Terms Of Service for many web sites, and to keep users from "ripping them off", web sites like YouTube change the way they deliver video files periodically to protect their property. When changes like that are made, the developers of those types of add-ons have to fix their stuff to get it to work properly again.

IIRC, that Video DownloadHelper extension has been broken 3 or 4 times in 2013 and has had to be fixed each time.

Probably the support site will be up tomorrow although it is possible that the site will down during the holidays.
I've noticed more often that the support site isn't available during the weekend.