Welcome to Trisquel!
In Settings->keyboard, you can find the shortcuts tab; you seem to have done
this. You'll notice categories of shortcuts, including 'launchers', this for
your default email, media player, media player controls, etc. To activate a
shortcut, highlight it, then move right, there, you can press the keys you
want to use for the shortcut; for instance, I have 'ctrl+super+left, right,
up,down', for previous track, next track, vol-up, vol-down. Also 'ctrl+alt+b
and m' for browser and mail, respectively. There is also a 'custom' option
for binding a shortcut to any other command you like.
Re: the search bar, are you referring to GNOME Shell? that can be installed
from synaptic package manager; the packages you want are gnome-session and
gnome-shell. On next login, you can choose Gnome session instead of
Trisquel.
What browser extensions are you looking for, and have you tried abrowser, as
well as icecat? Trisquel uses its own addons page that may not show as many
addons as on the FF page, because the Trisquel one will show only the libre
ones.
HTH,
Dave
