Some things you might want to add back, that Murder-zillla suppressed or destroyed in Firefox 26+.

The ability to block/unblock images and js (went missing from the Options panel).
https://addons.mozilla.org/en-US/firefox/…

Also, maybe you want an image block toolbar button:

https://addons.mozilla.org/en-US/firefox/…

The Downloads outboard panel that Fkup-zilla took away:
https://addons.mozilla.org/en-US/firefox/…

Maybe you don't want the hamburger menu. Put this into Stylish or userchrome.css.

/* AGENT_SHEET */
@namespace xul url(http://www.mozilla.org/keymaster/gate…);
#PanelUI-menu-button { display: none !important; }

Or maybe you want some of Ffx's UI fonts to be more in line with the page fonts you chose. I find that without this css, Firefox's fonts for forms/text boxes/etc are too small and don't match what I've chosen elsewhere.

/* Browser Fonts */
menubar, menubutton, menulist, menu, menuitem, textbox,
toolbar, .tab-text, tree, tooltip, sidebarheader, statusbar {
font-size: 16pt !important;
font-family: Tahoma !important; }

Hope you like.