How do I enable adblock on Firefox broweser?

http://www.mozilla.org/support/firefox/adblock

If you are referring to Adblock Extension, go to
https://addons.mozilla.org/extensions/moreinfo.php?application=firefox&id=10
and click Install Now link. A dialog box will show up, wait a few
seconds and click Install Now button. After successful installation,
you should restart your Firefox browser. For additional settings of
adblock, see menu Tools>Adblock>Preferences.

If you want to disable annoying Flash animated banners, go to
Tools>Adblock>Overlay Flash or press Ctrl+Shift+F keyboard
combination. It will change flash plugins into orange and you can
click and adblock them. Make sure you press Ctrl+Shift+F again or you
won't see Flash animations. To adblock a specific frame or picture,
right click on it, and select the 'Adblock Image' (or Adblock iframe)
on the bottom.

Dear patrice29,

The adblock extension is only as good as the filters you have set. 
Once you have the extension installed and working properly, go to
http://www.pierceive.com/, download the Filterset.G extension and
install the filters which are listed on that site.  The latest
filterset can be found at
http://www.pierceive.com/filtersetg/2005-09-18a.txt.  Once you have
downloaded this .txt file to your PC, open your Firefox browser and
proceed to Tools > Adblock > Preferences > Adblock Options > Import
Filters.  There, a dialog box will pop up asking you to select a file
to install the filters from.  Select the Filterset.G .txt file and
load it into Adblock.  Click "Done", and restart Firefox.  Welcome to
the world of browsing with no more ads!