I've been using full flat for three years, here's the tip for using this theme on newer version of firefox.
1. Use chrome or any browser to download .jar file on this page.
2. Open .jar file with Winrar and find install.rdf, open it with notepad.
3.Find 3.6.*, change 3.6 to the firefox version number you are using, or, just change 3.6 to 99.
4.Save install.rdf and update the .jar file.
5. Drag edited .jar file to firefox's add-on Manager then install.
6. Restart firefox.
I hope this help! and sorry for my bad English :P