You should be able to get it directly from firefox's ftp, extract it, and use
it. Add a link to it in the menu with preferences->main menu
http://releases.mozilla.org/pub/mozilla.org/firefox/releases/latest-4.0/linux-x86_64/en-US/firefox-4.0.1.tar.bz2
( use the same url, and go back a few directories if you want a different
version or architecture. )
As for installing packages that are not in Trisquel's repositories: I usually
use the command line:
sudo add-apt-repository ppa:mozillateam/firefox-stable
sudo apt-get update
sudo apt-get install firefox
I have not tried these commands for firefox personally, so I cannot say if it
will work or not in this case. But generally speaking the procedure for
installing any non Trisquel hosted software can be done that way: Find a ppa
that hosts the software package, add it, update, install.
For example, I have successfully installed icecat ( gnu project's version of
firefox 4 ) the same way:
sudo add-apt-repository ppa:gnuzilla-team/ppa
sudo apt-get update
sudo apt-get install icecat
abrowser is firefox 3.5-3.6 and icecat is firefox 4 with only a difference of
name, plugins installed by default, and branding.
ps: The title "Adding packages to repository" is not right in this case. You
want to add packages to your system from another repository; not add packages
to Trisquel's repository (what one might assume from your wording).
