If your firefox has forgotten about all the nice icons for button in the UI (i.e. if there is a back button but it has no label or icon and all the “close tab” buttons are just tiny little invisible buttons), this bug is for you: https://bugs.launchpad.net/ubuntu/+source/ia32-libs/+bug/641056
If you start Firefox from the console, you’ll see errors like this one:
(firefox-bin:30535): Gtk-WARNING **: Error loading icon: Unable to load image-loading module: /usr/lib/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-png.so: /usr/lib/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-png.so: wrong ELF class: ELFCLASS64