Among the most noteworthy options in the program are the possibility to change the different favicons of the websites you visit, modify the look of the Firefox button, change the location of the status bar, or see the size of a file when you try to download it.
In addition, the program can back up your browser information by choosing exactly which elements you wish to save. Passwords, preferences, AdBlock filters, add-ons ... just tick the box and it's done.
Mar Mod is one of the most comprehensive add-ons you can install on Firefox. It's not essential, but once you get used to it, it'll be hard to let go.