Rated 5 out of 5 stars

Just a quick update. A quick edit to the .rdf file in the extension to change the current "max" version to 3.5.* (or higher if you wish) allows this extension to work again (I did this and tested it and so far it's working just fine).
Click ignore version then:
Right click and save as (where you'd normally click to install extension) and download the "copy_plain_text-0.3.3-fx+mz+tb.xpi" file. change the extension to .zip then open that file with an application that can open .zip files (xpi is really just a renamed zip file). Extract the "install.rdf" file & open it with a text editor (eg word pad). and find the line that says: "2.0" and edit that to read "3.5.3.*" or a higher number if you wish (I used "10.0.0.*" lol) (edit the version number inside the file too) save the file and drag the edited file back to the "copy_plain_text-0.3.3-fx+mz+tb.xpi.zip" (you changed the extension earlier) and remove the .zip extension; change the version number to .4 if you wish or what ever. Then drag the .xpi file to a firefox window (or hit Ctrl+O & select the .xpi (extension)) and it will ask you if you want to install the add-on. Restart/configure and copy text away!!! :) Hope this was helpful.

PS Looks like somebody already posted this info awhile back; I didn't see it though.