Empty target directory instead of re-creating it

For better web-ext compatibility.

tools/make-firefox.sh

@@ -8,8 +8,8 @@ echo "*** uBlock0.firefox: Creating web store package"
 
 BLDIR=dist/build
 DES="$BLDIR"/uBlock0.firefox
-rm -rf $DES
 mkdir -p $DES
+rm -rf $DES/*
 
 echo "*** uBlock0.firefox: Copying common files"
 bash ./tools/copy-common-files.sh $DES