not able to watch flash video clips online

You need to install the flash plugin

flashplugin-nonfree

Use the graphical package manager or a terminal
"sudo apt-get install flashplugin-nonfree"

Just for the record.
Invalid Duplicate:
https://answers.launchpad.net/ubuntu/+source/yelp/+question/96519

In order to get flashplayer working and to avoid conflicts, make sure no other flashplayers are installed, like gnash, swfdec.
Also check that these packages aren't installed:
mozilla-plugin-gnash
swfdec-mozilla
https://help.ubuntu.com/community/RestrictedFormats/Flash

Make sure not to run more than one package management at a time,
e.g. Synaptic and Terminal (apt-get cmds) or Ubuntu-Software-Center and Update Manager.

Also verify which flashplayer is needed, either for 32bit or 64bit architecture.
This terminal cmd will tell you which architecture is installed:
uname -a

Firefox should be closed when proceeding an installation.
As suggested from Appiah, you may open a terminal window and
type (or better copy and paste to avoid mistyping):
sudo apt-get install flashplugin-nonfree

##It will ask for your (login) password; while typing it, no stars or alike will appear, you'll type blind.
Reference:
http://library.gnome.org/users/gnome-terminal/stable/gnome-terminal-get-started.html.en
https://help.ubuntu.com/community/UsingTheTerminal
https://help.ubuntu.com/community/RootSudo#Usage
https://help.ubuntu.com/community/AptGet/Howto

Via Synaptic search for flashplugin-nonfree, right click -> install, then press 'Apply' on the main menu.
https://help.ubuntu.com/community/SynapticHowto

Then restart Firefox.
To verify which version is used, type in url bar of Firefox:
about:plugins

Or go to Tools -> Addons -> Plugins