Undisputed Benefit of Search Engines

Well, this was proven or stated eons ago (internet age). But it's always a treat (ice cream anyone?) to find a solution to your problem by simply typing the question...and voila! Someone, somewhere, on this planet, had the very same issue, and hence the very same question -- before you.

I refused to update my Firefox since version 24 (30 is the last one as of today) simply due to the fact that it messed up my bookmarks sidebar's font. They went from size small to size XXL. What a weight gain! Perhaps, muscle due to unhealthy steroids?
Anyway, so I executed my troubleshooting process, which is to simply google my question. The first time I ran into the problem since Firefox 25, there was no real easy solution to it. By easy solution, I mean copy paste without changing my seating. But today, there are bunch of them! Thanks Google. Thanks curious developers!

Here is the best (easiest) way to decrease the spacing between /font of your sidebar bookmarks:
Source: https://support.mozilla.org/en-US/questions/977387#answer-501740

Helpful Reply

Yes, specific steps would be helpful, especially the first time.
Stylish will add an "S" icon either to the main Navigation Toolbar or to the Add-on bar. If you do not normally display the Add-ons bar, you can turn it on and off by pressing Ctrl+/ or right-click a blank area of the tab bar (or the new tab "+" button).
(1) Select and copy the rule:

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
.sidebar-placesTree treechildren::-moz-tree-row,
.placesTree treechildren::-moz-tree-row {
  height: 1.3em !important;
  border-width: 1px !important;
}

(2) Click the Stylish "S" icon > Write new style > Blank style. You should see something like the attached.
(3) Paste the rule in the lower area showing "1" at the left (a line number should appear for each line).
(4) Click the Preview button to apply the rule, then open (or close and re-open) the Bookmarks Sidebar (Ctrl+b or from the View menu) to force a re-draw. Success?
(5) Before saving the rule, type a name in the top box (for example: Bookmarks Old Vertical Spacing)
That's it. Unless there's a problem in Step 4.