Is this about vertical space or horizontal space?

Does this happen with the History menu or the history sidebar or are you opening the History Manager (Library)?

Note that you can drag the right border of the sidebar to reduce the width.

Can you attach a screenshot?

• https://en.wikipedia.org/wiki/Screenshot
• https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

• use a compressed image type like PNG or JPG to save the screenshot
• make sure you do not exceed the maximum size of 1 MB

If you mean the drop-down below the address bar when you click the triangle for frequently visited sites or while you are typing something:

There is no built-in setting to reduce the width. However, you can alter the appearance of that autocomplete list using custom style rules. Those rules can be applied to Firefox's interface using either (1) the Stylish extension (easier to experiment and revise) or (2) a userChrome.css file (if you don't want any more extensions).

In response to other user comments, I created a user style for the drop-down that you could use, or just use for inspiration to request your own custom style rules. More info here:

https://userstyles.org/styles/122394/url-bar-tweaks-remove-visit-search-limit-width

Yes, I want to use the old two-line drop-down. I downloaded the Stylish extension, but I don't know what URL that I need to use to install the two-line add-on. Can you assist me jscher2000?

Note that you can also place such CSS code in the userChrome.css file in the chrome folder in the Firefox profile folder.

I use code like this:

/* urlbar - separator */
#urlbar .dropmarker-icon, #urlbar toolbarbutton {border-left:1px solid ThreeDShadow !important}
#urlbar-icons image {border-left:1px solid ThreeDShadow !important}

/* urlbar - border */
.autocomplete-richlistitem {padding-top:2px!important; padding-bottom:2px!important}
.autocomplete-richlistitem:not(:first-child) {border-top:1px solid #ddd!important}
.autocomplete-richlistitem[selected="true"] {border-top-color:transparent!important}

/* urlbar - width */
#PopupAutoCompleteRichResult{
 width: 1000px !important;
 max-width: 1000px !important;
 margin-left: 0 !important;
}

• create the chrome folder (lowercase) in the <xxxxxxxx>.default profile folder if this folder doesn't exist
• use a plain text editor like Notepad to create a (new) userChrome.css file in the chrome folder (file name is case sensitive)
• paste the code in the userChrome.css file in the editor window
• make sure that the userChrome.css file starts with the default @namespace line
• make sure that you select "All files" and not "Text files" when you save the file via "Save file as" in the text editor as userChrome.css.
  otherwise Windows may add a hidden .txt file extension and you end up with a not working userChrome.css.txt file

You can use this button to go to the current Firefox profile folder:

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Thank you, cor-el. You made my day!