Rated 4 out of 5 stars
First, let me thank the Dev for making a great little add-on. I hope he doesn't get mad, but I see a lot of people asking about changing the icon or making the background transparent, so I thought I might chime in.
1) Install the add-on & restart firefox.
2) Disable the add-on via the add-on menu & restart firefox and then close firefox.
3) Select the firefox tab -> Help -> Troubleshooting Information. This will open a tab. Select "Open Containing Folder" This should open your firefox profile folder in Windows.
4) Open the "extensions" folder. All your add-ons & themes for the most part are named some random string... This add-on however, starts with "firefox-menu-icon@...xpi", alternatively you can also find the most recently created file in this folder, as it should be the add-on you just downloaded.
5) Copy the .xpi file somewhere you can work on it, like your desktop, and change the file extension to .zip
6) Open the .zip file with something like winrar (preferably) or winzip, etc.
7) The icon used with this add-on is labeled "default22.png" and is 22x22 in size. Choose a icon of your liking, re-size it, re-name it to match, and drag and drop into the .zip file. Make sure you choose to replace the file already in the .zip and also choose STORE as the compression method (you do NOT want to compress these files if you are replacing them in the .zip).
8) Once you're finished, change the file extension back to a .xpi
9) Open firefox, open the add-on manager, choose "Install add-on from file" and navigate to the file on your desktop or where ever the one you just edited is at. Once installed, enable it and restart firefox... should be changed and good to go.
These directions pertain to changing the background color/transparency are require a little more knowledge (First six steps are the same as above):
7) Unzip "menu-overlay.css" file and open it with your editor of choice (Notepad is default, I like Notepad++).
8) The main lines of interest are the "background-image: -moz-linear-gradient" rgba color codes & transparency percentages under
Change to your preference of color (changing them all to 0, will make it completely transparent).
Hope this helps!