Extension icons disappeared from toolbar
How to recover my extension icons that disappeared from the toolbar?
You can add buttons to the Firefox toolbar by pressing the menu button (three lines) in the top-right corner of the screen and selecting the Customize option from the drop-down menu. When you are done making the changes to the toolbar, you can close the customize tab. See Customize Firefox controls, buttons and toolbars for more information.
If your toolbar has a lot of items, some may be moved into the overflow menu, which will appear as two right-facing arrows on the toolbar. You can also use the customization mode mentioned above to move buttons in and out of the overflow menu.
Hope this helps.
Note that some extensions may not run by default in Private Browsing mode.