Well there was an initial kerfuffle trying to get the extension "firstxpcomchrome" working in my Minefield build. Turns out that when I was attempting to connect it up via a text file in the profiles/myprofile/extensions folder, I had put the path as a c:/.... path which, in Unix (ie Mac), is not correct. This was affecting my ability to install the extension through drag n drop because the file for email@example.com was not being overwritten so even though the extension appeared to install correctly in the add-ons manager window, upon restarting it was nowhere to be seen. Deleting the erroneous flat file and doing a drag and drop installation cleared this up and I was able to continue with the lab.
Aside from crashing the browser every time I called on "Reload All Chrome" from the Extension Developer add-on, the lab went smoothly.
(please enjoy the File menu behaviour that is happening in the background)