I created a firefox-spell-pt-br package to firefox3 (from [unstable]) and it is working. The firefox3 of [unstable] is still in /opt/mozilla. This is the only difference.

Below is the PKGBUILD:

# Contributor: Hugo Doria <hugodoria@gmail.com>

pkgname=firefox-spell-pt-br
pkgver=3.0b5
pkgrel=1
pkgdesc="Portuguese (Brazil) dictionary for Firefox"
arch=('i686' 'x86_64')
license=('GPL')
depends=("firefox3>=$pkgver")
makedepends=('unzip')
url="http://www.broffice.org.br/verortografico"
source=(http://www.deso-se.com.br/downloads/pt_BR-2008-02-21C.zip)
md5sums=('fbcab4da77624ec53b9b7846e452af12')

build() {
        cd ${startdir}/src

        # remove all unwanted files
        rm *.TXT

        install -D -m644 pt_BR.aff ${startdir}/pkg/opt/mozilla/lib/firefox-3.0b5/dictionaries/pt-BR.aff || return 1
        install -D -m644 pt_BR.dic ${startdir}/pkg/opt/mozilla/lib/firefox-3.0b5/dictionaries/pt-BR.dic || return 1
}