The maximum number of candidates to be displayed in the URL bar auto-completion feature can be changed in the about:config -> browser.urlbar.maxRichResults.
However, it was not possible to change the number to be displayed at once in the drop-down list has been fixed at 6 rows.
This addon is able to change max rows of autocomplete dropdown list on url bar.
Please try this addon. More rows will be displayed at once, so you have greatly improved operability.
The maximum number of rows that can be set in this extension is the same as "browser.urlbar.maxRichResults" the maximum number of candidates to be retrieved from the bookmarks and history.
browser.urlbar.maxRichResults default is 12, so, if you want to increase the maximum number of lines than line 12, please try from make yourself change the maxRichResults in advance.
Since Version 1.2
maxRichResults is now possible to change the options page and support "Suggestion Settings".
Since Version 1.3
Support "Disable moz-action:switchtab" for autocomplete in urlbar.(Default is FALSE)
"if you already have that site open. If you do, you’ll be directed to the existing tab so you don’t open a duplicate."
"The Future of Developing Firefox Add-ons | Mozilla Add-ons Blog"
"Add-ons/developer/communication - MozillaWiki - Upcoming Changes in Add-on Development"https://wiki.mozilla.org/Add-ons/developer/communication#Upcoming_Changes_in_Add-on_Development