As posted below, bit_dot_ly_slash_T3lwy3 has a workaround that forces Firefox to allow you to install this theme.
However, in the future Firefox will make even more radical changes to the Firefox GUI. For example, they plan to replace the entire download GUI with something else while moving it to the sidebar... which they will rename the "library"; This change will break the navigation toolbar buttons used with this theme. Oh, and it also forces you to not use a separate window for Firefox's download manager. So, once again, Mozilla isn't listening to us. And once again, we the people have fought back.
"Classic Toolbar Buttons" by Aris is a fantastic workaround (or at least it does in Nightly 21.0a1 (2012-01-09). I highly recommend it. Used in conjunction with the bit_dot_ly_slash_T3lwy3, it works beautifully.
To do it right:
1) Install the bit_dot_ly_slash_T3lwy3 fixed theme.
2) Install Classic Toolbar Buttons (google it)
3) Now go to Tools> add-ons> Extensions> Classic Toolbar Buttons > Options.
3) Scroll down to the "Other tweaks" section and check off all 5 boxes in that section.
4) Restart Firefox and enjoy your classic GUI.
5) Thank Aris, and thank the good Mr Morton who made this theme for us.
You can thank me too if you want. :)