fix build script

build-ff.sh

@@ -2,14 +2,19 @@
 PWD=$(pwd)
 BASE_DIR=${PWD}/base
 MANIFEST_DIR=${PWD}/firefox
-OUT="../out/artifacts/firefox/extension.zip"
+OUTPUT_DIR=${PWD}/out/artifacts/firefox
+OUTPUT_FILE=${OUTPUT_DIR}/extension.zip
 
 echo $BASE_DIR
 echo $MANIFEST_DIR
-echo $OUT
 
-rm -f ${OUT}
-cd "$BASE_DIR" || return
-zip -r ${OUT} *
-cd "${MANIFEST_DIR}" || return
-zip -r ${OUT} manifest.json
+mkdir -p ${OUTPUT_DIR}
+
+# check if file exists before removing
+[ -f ${OUTPUT_FILE} ] && rm -f ${OUTPUT_FILE}
+
+pushd ${BASE_DIR}
+zip -r ${OUTPUT_FILE} *
+popd
+pushd ${MANIFEST_DIR}
+zip ${OUTPUT_FILE} manifest.json