Hi Dad, it's unfortunately not a one-click operation. As far as I know, there isn't a built-in preference for this.

There is a community supported workaround, which is to custom style the user interface using a userChrome.css file. I posted the steps a few hours ago in the following thread:

https://support.mozilla.org/questions/1337841

Thanks, Jscher2000!

Using the link you provided I was able to create the userChrome.css file and then changed the padding block to 2px. Perfection! Thanks for saving my sanity!

All of a sudden my line spacing for bookmarks doubled. How do I revert to single spacing. Version 92 Firefox. you keep changing stuff that doesn't need to be changed

All of a sudden my line spacing for bookmarks doubled. How do I revert to single spacing. Version 92 Firefox. you keep changing stuff that doesn't need to be changed

This changed starting in Firefox 89. In Firefox 89-90 there was a temporary preference to defer the change. That was removed in Firefox 91. What have you been doing up to now?

Currently, there is no built-in way to tighten up the spacing. Making that possible is one of the most popular ideas on Mozilla's new "ideas" site: https://mozilla.crowdicity.com/post/720482

There is work under way to tighten up the bookmark menus that drop from the main and bookmarks toolbar. In Firefox 93 (next release), the optional Compact density setting will do that. Here's a preview from the "Developer Edition" (based on the beta test version of Firefox 93):

However, Firefox 93 isn't scheduled to release until early October.

Until then, what support volunteers can suggest is limited...

Unofficial Community-Supported Workaround

Firefox users can modify the user interface of the browser using custom style rules. This involves setting up a userChrome.css file. If this is your first time, make sure to set aside 10 quiet minutes to work through it.

The full instructions are in the following reply: https://support.mozilla.org/en-US/questions/1346577#answer-1435049