For Firefox, use Abrowser, as onpon4 said. The only differences are the
branding, an addons site that only lists free (as in freedom) addons, and
improved default settings (for privacy).
Trisquel includes LibreOffice, which was forked from OpenOffice.
For videos and audio, VLC is really good, and supports a lot of formats. Also
Amarok and Rhythmbox are great music players.
There is an antivirus in Trisquel called clamav that you can install, but
it's mostly for checking for Windows viruses. There's not much of a point in
using antivirus.
