Mozilla has added an 'osKey' modifier to input events, which apparently
corresponds to the likes of the Windows key.  Somebody suggested that
Conkeror treat this modifier as 'super' with the 's' prefix, and submitted
a patch:

https://github.com/ubolonton/conkeror/commit/79cbf379fccbb65c63a7debdab1e7dca110e67d0.patch

I want to find out exactly what version of the Mozilla platform introduced this osKey modifier, so that
the commit message and any comments in the patch can be accurate, and so that we can give good advice
to users that have problems related to this.

I updated Conkeror's contrib/xrev program to detect osKey, and then tried it with a range of XULRunner
versions.  My keyboard has an OS key which X sees as Super_R.  My findings:

XULRunner <= 12 report the Super_R key as the metaKey modifier.

XULRunner 13 and 14 do not report the modifier at all.

XULRunner 15 through 19 report it as an "OS Key", but not via the expected 'event.osKey' property, but
via event.getModifierState("OS").

resolved by a patch based on the one linked above