Try to reset the browser.uiCustomization.state on the about:config page if Restore Defaults on the customization page doesn't work.

Thank you for your answer. I wouldn't, however, want to lose all the other icons of my other addons, as well. Then I had to install all the other addons again, I suppose.

Resetting this pref only affects the customization of the toolbars. If you can't find some icons added by an extension then you can look in the Customize palette.

• "3-bar" Firefox menu button > Customize
• View > Toolbars > Customize (press F10 to show the Menu bar)
• Right-click empty toolbar area

• https://support.mozilla.org/kb/customize-firefox-controls-buttons-and-toolbars

Make sure that toolbars like the "Bookmarks Toolbar" are visible.

• "3-bar" Firefox menu button > Customize > Show/Hide Toolbars
• View > Toolbars (press F10 to show the Menu bar)
• Right-click empty toolbar area

Open the Customize window and set which toolbar items to display.

• "3-bar" Firefox menu button > Customize

• check that "Bookmarks Toolbar items" is on the Bookmarks Toolbar
• if "Bookmarks Toolbar items" is not on the Bookmarks Toolbar then drag it back from the Customize palette into the Customize window to the Bookmarks Toolbar

• if missing items are in the Customize palette then drag them back from the Customize window on the toolbar
• if you do not see an item on a toolbar and in the Customize palette then click the "Restore Defaults" button to restore the default toolbar setup

Thank you again, very much. My original point was, however, that these icons weren't even in the Customize palette, i.e. anywhere. That's exactly the problem.

I beg your pardon. All the above seems to have been caused by Firefox updating to v31. After I downgraded to v30 everything is normal as before. Updating is not recommendable until Firefox works normally.

Did you try to click the Restore Defaults button to see if that has effect?

There is an undo possible, so testing this is not much trouble.