This is in the Bookmarks Sidebar? I would think it would follow the same font size as the rest of the interface, but maybe that has become decoupled somehow.

There isn't a built-in way to modify the font size in the sidebar, but many aspects of Firefox's interface can be tweaked with custom style rules. Hopefully another Mac users will be along to assist with that approach, or I can try to see whether something that works on Windows might work on your system (but not tonight, sorry).

Same problem for me. This is serious enough for me that I may need to switch to another browser. I rely heavily on the bookmark sidebar to be able to quickly access my many hierarchical bookmarks and the huge font size makes quick access impossible. Haven't found anything relevant in Support except this forum thread. I assume an update caused the problem.

I'm running Mac OS X 10.9.5.

Hi charliemyboy, some other Mac users posted about changes to the line spacing in the Bookmarks Sidebar in Firefox 48, but this is the only thread I've seen so far about the font size.

Do you use any add-ons that modify the font sizes of the toolbar area or other non-content parts of the browser? Possibly it isn't applying to the sidebar in the expected manner. Otherwise, I would expect them to be the consistent (as usual).

You can apply custom style rules to various parts of Firefox's user interface using either the Stylish extension or a userChrome.css file. For example, the following rule specifies the font size for the Bookmarks Sidebar (12px, or 12 pixels, is the typical size of text in the URL bar):

/* Set font-size for Bookmarks Sidebar */
#bookmarksPanel {
  font-size: 12px !important;
}

If you're interested in trying that, I or someone can give more detail.

Thank you, thank you, jsher2000!

I originally thought the problem was both line spacing and font size, and posted here since this was the first relevant thread I found. In fact, your sidebar line spacing fix in thread https://support.mozilla.org/en-US/questions/1133496 did the trick. I might play with font-size too, but I think the line spacing was all that actually changed.

Hail to the Mozilla User Community--one more great reason for using Firefox!