Add GitHub Actions workflow for Firefox extension build

.github/workflows/main.yml

@@ -0,0 +1,31 @@
+name: Build Firefox Extension
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+
+      - name: Install dependencies
+        run: npm install
+
+      - name: Build XPI via makefile
+        run: make package
+
+      - name: Upload XPI artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: fileleak-xpi
+          path: out/fileleak.xpi