"no flash found on this page"

My thought is that message refers to not finding any Flash files on that webpage, and not anything to do with having Flash installed in Firefox.
Maybe the video on that YouTube page was using a different format, like maybe HTML5?

BTW, the Free Download Manager support forum is here:
http://www.freedownloadmanager.org/board/

I am sure youtube video is still on flash, since I just checked it. But I could download the video using a different plugin, flash and video download 1.46. The problem is that FDM plugin no longer works and I am not sure if it is caused by firefox or a problem of the FDM plugin?

Sorry, I can't help you any further, I don't use FDM or any other external download manager.

Have you tried the FDM support forum yet?

I post the same question FDM forums but has not receive any solutions

For others reading, here's the "Free Download Manager" forum thread where Alphas posted his question:

• http://www.freedownloadmanager.org/board/viewtopic.php?f=1&t=16047&start=45 PLEASE put up all FDM and Firefox problems here!

Alphas: If you don't get an answer on the "Free Download Manager" forum then you could contact http://www.freedownloadmanager.org/support.htm to report the problem.

You could also disable the "Free Download Manager Firefox add-on from the Firefox -> Add-ons _> Extensions list and try a different extension to download Flash content. You can look for alternate download managers at addons.mozilla.org

All FDM (Free Download Manager) Problem Solutions :-

Follow steps it works

1.Download add-one "Download-Helper" visit https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/?src=userprofile

2.play any video from any site suppose from youtube ex.http://www.youtube.com/watch?v=bgzvywZMZq0

3.Just Drag Mouse on that DownloadHelper Button then if you select 720p video copy just Url of that video

This is URL provided by Download Helper that we have to paste in "FDM Add Download Button" for above video "http://r4---sn-cvh7zn7d.googlevideo.com/videoplayback?id=6e0cefcb064c66ad&ratebypass=yes&upn=lP80umpFR_s& ipbits=0&mt=1391454022&gcr=in&expire=1391478143& sparams=gcr%2Cid%2Cip%2Cipbits%2Citag%2Cratebypass%2C source%2Cupn%2Cexpire&itag=22&ip=14.97.150.199&key=yt5&sver=3& ms=au&source=youtube&mv=m&f exp=939913%2C906950%2C910207%2C929305%2C936910%2C936913& signature=EEA38453B1A800E3B657B5C32097016F682E4DDD.6922A0C 0A729D0368B3AAA8788718FAD4F5C6697"

4.Paste in FDM 3.9.3 build 1360

5. Enjoy Downloading Video Free.

For Resume After stop of video   :- 

1.Go To that page from where u downloaded video suppose ex.http://www.youtube.com/watch?v=bgzvywZMZq0

2.Just Drag Mouse on that DownloadHelper Button then if you select 720p video copy just Url of that video

This is URL provided by Download Helper that we have to paste in "FDM Download Properties" for same video

if video fails resume after some time again go to youtube site copy 720p url & paste in download properties of "videoplayback" in FDM

"http://r4---sn-cvh7zn7d.googlevideo.com/videoplayback?key=yt5&ip=14.97.150.199&upn=oDWByiyx69g&mt=1391454784& sparams=gcr%2Cid%2Cip%2Cipbits%2Citag%2Cratebypass%2Csource% 2Cupn%2Cexpire&source=youtube&ratebypass=yes&ipbits=0&mv=m&gcr=in& itag=22&id=6e0cefcb064c66ad&ms=au&fexp=933513%2C924615%2C902006% 2C932280%2C916807%2C926515%2C916615%2C936910%2C936913&sver=3& expire=1391478143& signature=9D54EBDA3B92575D6396E79A4976F593831 ABDD6.4A2F306F4AEF7AE4673138A00E03EDC3679649E2"

3. Click on Start Download Button

Tested in Mozilla Firefox 26 & FDM 3.9.3 build 1360

It is possible that YouTube uses an HTML5 media player and not the Flash plugin.
You should be able to check that by right-clicking the media player.

It is in Flash and not HTML5