Or, alternatively, you can put
about:addons in the Firefox
evernotein the "Search all add-ons" field and hit enter.
After the add-on has been installed, you should see an icon with a red circle next to an elephant head appear slightly to the left of the menu button (the one with the three horizontal bars on it) and a tab should be opened in Firefox alowing you to log into your Evernote account. You can click on the icon with the elephant head next to the red button to save web pages to your Evernote account.
You should also now see "Evernote Web Clipper" listed in the extensions, if you click on Extensions from the Add-ons window.