use xpi instead of zip for webext

.travis.yml

@@ -3,7 +3,7 @@ env:
   matrix:
   - BROWSER=chromium EXT=zip
   - BROWSER=firefox EXT=xpi
-  - BROWSER=webext EXT=zip
+  - BROWSER=webext EXT=xpi
 script: ./tools/make-${BROWSER}.sh all
 deploy:
   provider: releases

tools/make-webext.sh

@@ -31,7 +31,7 @@ python tools/make-webext-meta.py $DES/
 if [ "$1" = all ]; then
     echo "*** uMatrix.webext: Creating package..."
     pushd $DES > /dev/null
-    zip ../$(basename $DES).zip -qr *
+    zip ../$(basename $DES).xpi -qr *
     popd > /dev/null
 fi