Hi, to get the icon to show on your screen on Firefox, just do the following:

1 - Click the puzzle piece icon, located on the top right of your screen.
2 - Find Adblock Plus.
3 - Right-Click on Adblock Plus.
4 - Select Pin to Toolbar.
5 - The icon now appears on the toolbar.

If those steps don't help, please submit a support request and we'll be happy to assist you: https://help.adblockplus.org/hc/en-us/requests/new.