Grab the bar shown here, and move it around.

Editing the "Name" of each bookmark is one way to get more bookmarks on the Bookmarks Toolbar.

Or hide the Labels using this extension. https://addons.mozilla.org/en-US/firefox/addon/only-icons/

Firefox has a predefined maximum width for bookmarks of 13em (which is the width of 13 m characters). If the bookmark label exceeds that width, the last character changes to ... to show that the text was truncated.

You can apply custom styles rules to various parts of Firefox's interface to override these built-in rules, so if you want to truncate the bookmarks names after a shorter amount of text, that would be another possible approach.

Sample style rule:

toolbarbutton.bookmark-item {
  max-width: 10em !important;
}