I downloaded it from http://linux.softpedia.com/progDownload/Copy-Plain-Text-Firefox-Download-21892.html It says Linux, but it works equally well on Windows 7. Click on "xpi (Softpedia Mirror RO)". Don't accept the automatic download, that will only try and fail to install it. Instead, right click on "click here" and chose "save link as". I then opened it in 7-zip, right-clicked on install.rdf and chose "edit". In the opened file, look for "em:maxVersion=" and change it to em:maxVersion="7.*". Close the document, and when you get the dialog "File 'install.rdf' was modified. Do you want to update it in the archive?", choose OK. Then in FF, choose open file and find the copyplaintext-0.3.5.xpi where you saved it on your hard drive. It will install like any other addon. I used the same procedure for FF5 and 6, and it worked fine in those versions, just as it does in FF7, at least so far (two days).
Oh, and if you have the old version still installed, you'll have to begin by removing it.