1. move existing firefox for backup
mv /usr/lib/firefox /usr/lib/firefox-old
2. download latest firefox for openSolaris. you can use below link for 3.6.3 version. please check for newest version
wget http://3347-mozilla.voxcdn.com/pub/mozilla.org/firefox/releases/3.6.3/contrib/solaris_tarball/firefox-3.6.3.en-US.opensolaris-i386.tar.bz2
3. extract it
bzip2 -d firefox-3.6.3.en-US.opensolaris-i386.tar.bz2
tar -xf firefox-3.6.3.en-US.opensolaris-i386.tar
4. move it to use.
mv firefox/ /usr/lib/
have a nice day.