Approximately 1 year ago Mozilla added a button to certain suggestions in the URL bar. This breaks the ability to tab through results as the focus is placed on the button instead of the item, requiring you to sometimes double tab.

Google Chrome and its variants also do this but there’s no workaround for them as they’re browsers that can charitably described as garbage.

You can fix this with Firefox by setting the following preference to false:

browser.urlbar.resultMenu.keyboardAccessible

No restart required, takes effect immediately.