Rated 3 out of 5 stars

In addition to what I typed below, to get an icon to show up in firefox 4 & 5

Once extension downloaded to desktop, open .xpi file
Go to folder “chrome\duplicatethistab.jar\skin\modern\duplicatethistab\”
Extract file labelled "tab-drag-duplicate-indicator.png"
Edit in a suitable programme that can resize by pixels
Resize image to 48x48 pixel, & save as Icon.png
Resize ORIGINAL file to 64x64, & save as Icon64.png
Merge back into .xpi file & drag into firefox to install
Sourced with help from http://blog.mozilla.com/addons/2010/12/02/toolbar-buttons-in-firefox-4/

This review is for a previous version of the add-on (1.2.1-signed.1-signed).  This user has other reviews of this add-on.