You can install LibreJS directly using a generated librejs.xpi file, or by building it from source.
jpm, you should be able to use the
command to build LibreJS from source.
make, a new file, librejs.xpi should be
generated. This is the file that can be installed in a Mozilla browser.
To install the add-on for all users, run:
sudo make install
or as root:
Next time you open a Mozilla-browser as a user of your system, you should be notified that a new add-on (in this case, LibreJS) as been installed and whether to allow it to run or not.