Add Firefox support and update cross-browser documentation

Expanded extension support to include Firefox alongside Chromium browsers. Updated README with detailed installation, build, and troubleshooting instructions for both Chromium and Firefox, clarified browser compatibility, and improved cross-browser guidance throughout code comments and documentation. Minor terminology changes were made in scripts and workflow files to consistently refer to 'Chromium' instead of 'Chrome/Chromium'.

.github/workflows/publish-extension.yml

@@ -182,9 +182,9 @@ jobs:
           body: |
             ## 📦 扩展包下载
 
-            ### Chrome/Chromium
+            ### Chromium
             - **文件**: `Xget-Now_${{ steps.version.outputs.version }}.chromium.zip`
-            - **支持**: Chrome、Chromium、Microsoft Edge 等基于 Chromium 的浏览器
+            - **支持**: Chrome、Microsoft Edge 等基于 Chromium 的浏览器
             - **Manifest**: v3
 
             ### Firefox
@@ -250,6 +250,6 @@ jobs:
           echo "📖 安装指南: https://github.com/${{ github.repository }}#installation"
           echo ""
           echo "📁 发布的文件:"
-          echo "  - Xget-Now_${{ steps.version.outputs.version }}.chromium.zip (Chrome/Chromium/Edge)"
+          echo "  - Xget-Now_${{ steps.version.outputs.version }}.chromium.zip (Chromium)"
           echo "  - Xget-Now_${{ steps.version.outputs.version }}.firefox.xpi (Firefox)"
           echo "================================"