Rated 4 out of 5 stars
Very useful! It's quite easy to use the 32 bit version of IE:
1) Download the add-on's XPI file by right clicking the "Add to Firefox" button and choosing "Save Link As"
2) Open the XPI with your favourite .zip editor (e.g. 7zip)
3) Edit open_in_ie-1.2-fx-win32.xpi\content\overlay.js
4) Change the path to the Internet Explorer executable from "C:\\Program Files\\Internet Explorer\\iexplore.exe" to the 32 bit one at "C:\\Program Files (x86)\\Internet Explorer\\iexplore.exe"
5) Update the XPI with the new file and open it in Firefox to install :)