I'm using myibay with Mac OS X and Firefox. I also have the Ebay toolbar.

There was a time when I had a "my snipes" button on the toolbar, but it has disappeared. This button was a link to all of the items that I was sniping.

I don't know when or how, but it's gone. Any idea why?

The Myibay add-on for Firefox never had any toolbar options. You probably added it manually.
You can add https://www.myibay.com/main on your toolbar shortcut.