As there is not much hope for a mozilla (security) upgrade in sarge I have built 1.7.11 myself. I started with the diff for 2:1.7.10-1, adapted the Build-Depends (sarge has no libxau-dev but xlibs-static-dev) and dropped the patch needed for building with freetype2 2.1.10-1 but breaking building against freetype2 2.1.7-2.4.
Shortly afterwards I thought the effort had been wasted but although DSA-775 refers to mozilla it just includes a fix for mozilla-firefox.
The package works for me, but I am only using the browser-part without
extension packages. If you are adventurous feel free to use them.
deb http://downhill.aus.cc/debian/ sarge mozilla