It still works, but you need Firefox ESR 32-Bit to use this add-on. It is ESR 32-bit, NOT ESR 64-bit, NOT Quantum. (If you need IE for some reason, you will also need ESR 32-bit to run Java & flash anyway.)
No longer works with Firefox 53.0.2 64 bit, can't use IE version (that is the last one for windows 7) on my system. I suggest using "Open in IE" instead as it works and you don't have to download anything extra.
I just found out this was updated and suddenly stopped working.I went back to the previous version to test and checked to stop updating as it solved the not working issue.I might try updating again if another version comes out compatible with Firefox v39.
Have been getting this error for the last couple of versions, which is annoying. The add-on still seems to work, at least for my purposes, though but it would be nice to see the developers fix this issue