There isn't a built-in option for this. Before Firefox 57 when add-ons could change the toolbars, you could do this with an add-on, but since then, you've needed to use custom style rules in a userChrome.css file. I don't know of the best rule set to use, but if this all sounds very new, I can link you to some resources:

(1) My site has background on userChrome.css style hacking. https://www.userchrome.org/

(2) The r/FirefoxCSS subreddit has many style wizards who maintain current sets of rules. https://www.reddit.com/r/FirefoxCSS/

Also, Mozilla has a product suggestion site at https://connect.mozilla.org/. I searched for whether anyone has posted this and didn't find an exact match. This was the most similar:

• Bookmarks toolbar show only icons

You can start a new "Idea" post to gather support for other proposed changes.

Thanks!!