Go to the Mozilla Add-ons Web Page {web link} (There’s a lot of good stuff here) and search for what you want.

There is also; Vertical Toolbar {web link} Adds a vertical toolbar at the edge of the window.

There are probably a lot of possibilities but a couple to try. Right click on a space in the toolbars. Make sure Navigation Bar and Menu Bar are both checked. You will also see Add-ons Bar; check that and a bar at the bottom of the screen will accept add-ons. I hope that in Customize(bottom of that page) you have set buttons to only show icons and not text. Hovering over the icons if you are not familiar with them will show the text. I hope this doesn't sound patronizing but those asking the questions here have a very wide range of experience/knowledge.

Feedback? Have you made any progress?

A number of responses require the installation of Classic Theme Restorer which I said I would not do because it's an incredible resource hog. I also do not want a vertical toolbar (aesthetics are important). I did discover accidentally that add-ons can be attached directly to the Tab bar which did solve my dilemma.

Note that you can place button on all toolbars including the Menu bar and the Bookmarks Toolbar and the Tab bar.