Cannot install jsprint extension in Firefox for MP Checkin2

I had to wipe and reload the OS on one of our check in kiosks. We are still running MPCheckin2, and have to use Firefox 56 for check in to work.
When I tried to search for the jsprint extension to install it in Firefox, search didn’t find it.
I manually downloaded the zip file from github, but it didn’t contain the .XPL file to manually add the extension.
Has anyone ran across this and have a way to copy the jsprint extension in Firefox for MPCheckin 2?
We’re upgrading to the new checkin in a couple weeks, but we really need this assisted station working for this Sunday, December 23rd.

I was able to copy the jsprint setup extension .XPI file from another working kiosk, and after a restart of the machine, we were able to print tags.
It doesn’t allow you to suppress the print dialog box, that option is greyed out, but at least it can print tags. In case anyone else still running MP checkin2 experiences this.
We’re upgrading in within the next 3 or 4 weeks so this will get us through until then.