According to me, the easiest way is to rename each bookmark with a shorter name (or delete the whole name if the website is recognizable with icon only). To do that, right-click on a bookmark > Properties, then change the text in the "Name" field.

You can restyle the Bookmarks Toolbar using custom style rules, for example, to hide the site icon or change the font. However, if you want full control of the bar, you may want to use an add-on. For example:

https://addons.mozilla.org/firefox/addon/roomy-bookmarks-toolbar/

Check this out;

Multirow Bookmarks Toolbar Plus {web link} Display your Bookmarks Toolbar in more than one row! Be smart about hiding text and only showing favicons, displaying website info as tooltips. Auto hide the Bookmarks Toolbar.

Multirow Bookmarks Toolbar Plus web site {web link}

You can also consider to organize the bookmarks in folders.

• https://support.mozilla.org/kb/bookmark-folders-organize-your-bookmarks

Add code to the userChrome.css file below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

#personal-bookmarks .bookmark-item:not([container]) .toolbarbutton-text { display:none!important; }

The customization files userChrome.css (user interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.

• http://kb.mozillazine.org/Editing_configuration