The add-on was removed by it's author, meaning they, for whatever reason, felt it should be removed. So you it may work when you update, but if it doesn't you should be able to find an alternative at addons.mozilla.org, for example, https://addons.mozilla.org/en-US/firefox/addon/multirow-bookmarks-toolbarplus/ might work.

You shouldn't remain on old and insecure versions of Firefox just for an add-on, it leaves you and your data at risk.

Quite frankly it should not be an add-on but a feature. But, that has been stated many, many times before now. Thanks for your answer but Not what I'm looking for.