Firejail will certainly be in Trisquel 8 because it will be in Ubuntu 16.04:
http://packages.ubuntu.com/xenial/firejail
If you cannot wait, download the DEB made for the architecture of your system
(the 'arch' command would tell):
32-bit:
http://downloads.sourceforge.net/project/firejail/firejail/firejail_0.9.38_1_i386.deb
64-bit:
http://downloads.sourceforge.net/project/firejail/firejail/firejail_0.9.38_1_amd64.deb
With GDebi (in Trisquel's repository), double-clicking on the DEB package
will install it.
