In my case, limited space on root partition is an additional problem. In the same time I have another, much bigger partition attached.
To address both issues I prepared this small script. It updates require dependencies (autoconf, sqlite, libpng, nspr, nss) and then updates Firefox, but with working directory in no default location (PKGMK_WORK_DIR).
prt-get update autoconf prt-get update sqlite3 prt-get update libpng prt-get update nspr prt-get update nss PKGMK_WORK_DIR=/media/pictures prt-get update firefox