.but after i double clicking on the package and authorising the installation,
"Double-clicking" "authorising", how did you do it? Usually "Flash" does not publish their plugin in .deb, .rpm, packaging formats. They offer it as a "run" script.
1. Check first if you are getting the correct 'flash*.gz' file; the website offers many including those intended for Mac and Windows. Get the one for Firefox 'Linux'.
2. Unpack the archive, it will create a folder containing the script. then open a terminal, you will 'cd' command toward that folder that flash created. Being there you will issue this command:
kangkan@host--$ sudo ./flash*.sh
Then the script will install by itself. Issue this again on terminal:
kangkan@host--$ /usr/bin/firefox restart
When firefox is restarted you can view the youtube.
3. Another way to install, this is easier: since you run Ubuntu it is easier for you to install flash online: click Menu > Admin > Synaptic. There you can install flash for Ubuntu Firefox.
Hope this helps.