The combined Stop/Reload button would appear at the left end of the Navigation Toolbar next to the Back and Forward buttons in current Firefox releases. The Go button is still at the end of the location/address bar.

See:

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

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

• 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.

Yes, thanks. But WHY did this disappear? I made no changes to my settings.

They haven't disappeared, but have been moved.

This change happened quite some time ago, so did you recently update from an older Firefox version?

Did you find this button?

This button is no longer fixed, but can be customized and moved to a different position on the Navigation Toolbar.