This is wonderful news to a user of Palemoon/basilisk. However
installing it failed because of my ignorance:

$ sudo apt-get install ./iceweasel-uxp-dev
Reading package lists... Done
E: Unsupported file ./iceweasel-uxp-dev given on commandline

$ sudo apt-get install ./iceweasel-uxp-dev_2.9-1~3.0gnuinos1_amd64.deb
Reading package lists... Done
Building dependency tree
Reading state information... Done
Note, selecting 'iceweasel-uxp-dev' instead of
'./iceweasel-uxp-dev_2.9-1~3.0gnuinos1_amd64.deb'
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
iceweasel-uxp-dev : Depends: iceweasel-uxp (= 2.9-1~3.0gnuinos1) but
it is not installable
E: Unable to correct problems, you have held broken packages.

I tried full path and adding version and .deb extension.

--
Haines Brown