Once you enabled the Addons, it make use of automatically.

https://addons.mozilla.org/en-US/about

See Customize Firefox controls, buttons and toolbars article,
drag it from toolbar to toolbar customization section, and then drag it to addon bar!

I looked on my desktop for add on there - they appear in the 'tools' section. The ones downloaded to my new laptop are not listed in that place

Open the Customize window to set which toolbar items to display:

• View > Toolbars > Customize
• Firefox > Options > Toolbar Layout

• Check that the "Bookmarks Toolbar items" is on the Bookmarks Toolbar
• If the "Bookmarks Toolbar items" is not on the Bookmarks Toolbar then drag it back from the toolbar palette in the customize window to the Bookmarks Toolbar

• If missing items are in the toolbar palette then drag them back from the Customize window on the toolbar
• If you do not see an item on a toolbar and in the toolbar palette then click the "Restore Default Set" button to restore the default toolbar set up

You can check for problems with the localstore.rdf file if this didn't help.

• http://kb.mozillazine.org/Corrupt_localstore.rdf

did a test run of this suggestion on my desktop PC - all worked as detailed in answer. Was able to add/remove icons easily. The icons were also on the view/tools list of options.

On my lap top when I select  tools/add ons   the add ons are listed (showing what the icon looks like too)  the enable/disable box read 'disable' for both add ons

When view/toolbar/customize is selected the add on icons are not present in the box nor on the tool bar

I reset toolbars to default setting with no effect