Firefox shows eight top sites in the location bar drop-down list when this is enabled for the Firefox Home page.
You can disable top sites via "Options/Preferences -> Home".

If you never want to see the drop-down list opening then you can set this pref to '0' (zero) on the about:config page.

• browser.urlbar.maxRichResults = 0

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

If you like other aspects of the address bar drop-down other than the automatic-opening list, you can disable the automatic opening behavior individually using:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste URLBAR and pause while the list is filtered

(3) [Firefox 75-77] Double-click the browser.urlbar.openViewOnFocus preference to switch the value from true to false

(3) [Firefox 78-79] Double-click the browser.urlbar.suggest.topsites preference to switch the value from true to false

In Firefox 78 and later you should be able to do this via Options/Preferences. The location bar drop-down list can show history items, bookmarked pages, pages opened in tabs, and search suggestions. You can set what suggestions show when you enter an URL in the location/address bar bar.

• Options/Preferences -> Privacy & Security
  Address Bar: When using the address bar, suggest:
  Browsing history
  Bookmarks
  Open tabs
  Top sites