You can try this code to the userChrome.css file below the default @namespace line. You can adjust the height and margin-top settings to suit your needs.

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

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

/* Line height for treechildren */
:-moz-any(#bookmarks-view, #historyTree, #editBMPanel_folderTree, #placesList, #placeContent)
treechildren::-moz-tree-row {
  margin-top: -1px !important;
  min-height: 14px !important;
  height: 14px !important;
}

You need to create the chrome folder (lowercase) in the <xxxxxxxx>.default profile folder if this folder doesn't exist.

• use a plain text editor (Windows: Notepad; Mac: Textedit) 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 the userChrome.css file starts with the default @namespace line

If you are on Windows:

• make sure 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.css.txt file.

• make sure you do not have a double .css.css file extension (userChrome.css.css.css) if it is still not working

I'm on a Mac. I had tried this already and it didn't work. I tried adding the code from above, but closed Firefox and reopened it and it still didn't work.

I think I have it in the right place. (username)/Library/Mozilla/chrome/userChrome.css. I don't understand why it's not working.

@ Barubiito

You need to create the chrome folder in the Firefox profile folder and place userChrome.css in this folder.

• ~/Library/Application Support/Firefox/Profiles/<profile>/

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

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

OK, now it works, thanks! I see now that I skimmed the linked page about the Profile folder too quickly and fundamentally misunderstood it. I read "profile folder" as being my user profile on my Mac, and took "~/Library/Mozilla/" to be where I should put the Chrome folder.

Thanks again.