Hi, the button has the label Email Link if it's available in the Customize window. See:

Customize Firefox controls, buttons and toolbars

If you spot it there, you can drag it to a location on the toolbar (or the overflow list if your toolbar is really full).

Any luck?

By the way, as a backup, you can use the File menu:

Alt+F to open the menu, then Email Link

Thank you very much for answering my question. I appreciate it.

I followed what you have told me even though I remember seeing the same screen and I found it on the bookmark bar. I never thought it is on a bookmark bar as it was located totally different spot before as far as I remember.

Thank you again.

Thank you for reporting back. I can't explain why the button moved, but you should be able to move it wherever you like in Customize mode.