Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Tools > Options > Privacy > Cookies: "Show Cookies"

Try to delete the webappsstore.sqlite file in the Firefox Profile Folder to clear the current local storage aka 'super cookies'.

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder

Sorry I did not get back to you sonner I did what you replied to my problem I did what you told me except my firefox profiles Here is a screen shot are these the files I should delete?

Thanks Joe

If Firefox is closed then you should only see the cookies.sqlite file and not the other temporary sqlite files.
Deleting that file removes all cookies, so you might want to try to clear cookies from affected domains first.

Did you already tried this and also removed the webappsstore.sqlite file?

I moved the file to my desktop so I can remove just the cookies I what to not all Trying to open the file boy haveing problem opening the file

Any help on that?

Don't try to open the SQLite file. Only Firefox is able to read the content of that file properly and show the content in the cookie manager.

Just try if it work better if the file is removed.

I moved the file to my desktop should I deleat it its 2 megs also still can't download from youtube

Are you talking about the cookies.sqlite file?

Yes, you can delete the file if you do not need to restore those cookies.

You seem to have gathered a lot of cookies if the cookies.sqlite file is 2MB.
You might consider changing the cookie settings and disable third-party cookies and let the cookies expire when you close Firefox.
You can set an allow exception if you want to keep cookies from specific websites like special settings or cookies that remember you (keep you logged in for a longer period).

• https://support.mozilla.org/kb/Enabling+and+disabling+cookies
• https://support.mozilla.org/kb/disable-third-party-cookies
• https://support.mozilla.org/kb/Cookies

Tried doing all that. Doesn't work. Still getting "0 KB". I think Downloadhelper is incompatible with YouTube now.

On Wednesday evening downloadhelper worked then on Trusday same as before

Help

in status bar I get download from >>http://o-o---preferred---sbb-beg1---v8---lscache3.c.youtube.com/videoplayback?upn=QAZ-Xt2bLxg&sparams=cp%2Cgcr%2Cid%2Cip%2Cipbits%2Citag%2Cratebypass%2Csource%2Cupn%2Cexpire&fexp=906016%2C900304%2C924104%2C901203%2C920917%2C922401%2C920704%2C912806%2C913419%2C913558%2C913556%2C912706&ms=au&expire=1347649893&itag=22&ipbits=8&gcr=rs&sver=3&ratebypass=yes&mt=1347624554&ip=178.148.33.118&mv=m&source=youtube&key=yt1&cp=U0hTTFZPV19OTUNOM19OSUFJOl9CczhRWWYtRk52&id=acb709c137505511&newshard=yes<<

now I see it's diffrent from, other downloads of video (for example it dosent start with >>o-o---preferred---sbb-beg1---v8---lscache3.c.youtube.com<< , btw sbb is the name of my internet provider),....my problem started when I unppluged my internet cable form my computer and I (with unppluged cable tryed to download),....once I had a similar problem with my homepage

I am having the same issue. Yesterday, I cleared the cache and removed the cookies as well as making sure to accept 3rd party cookies. That seemed to work for awhile but late last night, it started the 0 byte files again and continues today. I also removed the webappsstore.sqlite file but still will not work. I can download the videos with an external downloaded (Youtube Downloader HD) so the file can be downloaded. This looks like youtube changed something that DownloadHelper needs to fix.

Just recieved this from downloadhelper

From version 4.9.9 to 4.9.10

   After some changes in YouTube on September 14th, Video DownloadHelper was not able to download any video from this site, except the WEBM version while using the YouTube HTML5 mode. This is fixed in this addon version.
   After some changes in YouTube on August 6th, a number of users reported that all downloads were failing. This was caused by a a Youtube-generated cookie. The workaround consisting of clearing the cookies before downloading was not very convenient. This addon version takes care of that issue.
   Earlier this year, some other changes at YouTube caused the FLV videos downloaded through the "network" method to have a fixed 1.7MB size and being unplayable. This is now taken care of.