Yeah I had posted a long, detailed explanation in the ThinkPad thread, but I think it got deleted.
Short summary: Every Firefox theme is created differently. So there's really no universal method to fix every theme for W8Classic. What it comes down to is editing the theme's CSS and removing any styling for Windows Aero. There are two versions of the default Firefox theme: Standard, and Aero. The Aero version obviously contains the styling for Aero/DWM windows, while the Standard version contains styling for windows without composition (such as basic and classic windows). All I had to do to fix the default Firefox theme for W8Classic was package the Standard version of the default theme into its own xpi file.
Oh wow, well that doesn't look too good. I'll see if I can fix it.
However, I fear that whatever I do might be a short-term fix. Starting with Firefox 57, Mozilla will be killing off XUL extensions in favor of Google Chrome style webkit extensions, meaning that addons like Classic Theme Restorer are going to be unusable. Due to the nature of webkit extensions, it is unlikely that an alternative can ever be created.
Now, while most Firefox themes are not based on XUL (if any are) I am currently unsure if Mozilla plans to continue supporting complete themes. We'll just have to wait and see.
I probably just have to package Firefox 55's non-Aero theme into its own XPI file. I'll do that when I get the chance, but I'll leave the original version of the theme up for anyone who wants it. If there are still issues with it, I'll see what I can do to fix them.