If you have a lot of buttons, and the "House" button was toward the right end, it might have moved to an overflow list. That list has the >> button to drop down extra icons.

If that's not it, you can reset your toolbar button layout using the "Restore defaults" button on the Customize tab. More info in this article: Customize Firefox controls, buttons and toolbars.

Does that help?

You may have accidentally removed button(s) from the Navigation Toolbar.

If you right-click a toolbar item in normal (non-customize) mode then you can remove this item from the toolbar.

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

Open the Customize window and set what toolbars (Show/Hide Toolbars) and toolbar items to display.

• Right-click empty toolbar area -> Customize
• "3-bar" menu button -> Customize
• View -> Toolbars
  *you can tap the Alt key or press the F10 key to show the hidden Menu Bar temporarily

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

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