Have you read this KB article? https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars#w_how-do-i-customize-or-rearrange-toolbar-items

Each icon in the Customize Palette is supposed to have a text title. Are those text titles missing in your Firefox installation? If not, which text titles are you unsure of what they do?

See this support article:

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

• 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

You can right-click a toolbar item in normal (non-customize) mode to (re)move items.

• Select "Remove from Toolbar" to remove the button from the Navigation Toolbar and move it to the Customize palette ("3-bar" Firefox menu button > Customize).
• Select "Move to Menu" to move a button to the "3-bar" Firefox menu button drop-down list
• Select "Move to Toolbar" in the "3-bar" Firefox menu button drop-down list to move the button to the toolbar.

A title doesn't explain it. When I click on the icon nothing happens. What does "Open File" mean? Why does the icon exist? Which kind of file are we talking about about?

I clicked on the icon you mentioned and adobe flash crashed.

You realize that the rest of the explanation was gibberish without that information? All I see are icons with a title in a window. I don't see anything relating to any of the icons in the toolbars.

You need to click on the blue text in our posting to open the KB page that both us posted. Are you reading and watching / listening to the video on the KB page that webpage?

You need to click and drag the icons that are in the left side of the Customize window, and move them to either the right-side of the Customize window in the "Menu Panel" or onto one of the Toolbars to be able to use those icon / buttons - as shown in the video and in the image below that video. The icons on the left side aren't active buttons.