Add Firefox beta

emsdk.py

diff --git a/emsdk.py b/emsdk.py
index 164f865..82cc2e1 100644
--- a/emsdk.py
+++ b/emsdk.py
@@ -1291,7 +1291,7 @@
     firefox_version = os.path.basename(scraper.filename).split("firefox-")[1].split(".en-US")[0]
 
   print('Target Firefox version: ' + firefox_version)
-  if tool.version in ['latest', 'latest-esr', 'nightly']:
+  if tool.version in ['latest', 'latest-esr', 'latest-beta', 'nightly']:
     pretend_version_dir = os.path.normpath(tool.installation_path())
     orig_version = tool.version
     tool.version = firefox_version

emsdk_manifest.json

diff --git a/emsdk_manifest.json b/emsdk_manifest.json
index 1e468de..765ecd6 100644
--- a/emsdk_manifest.json
+++ b/emsdk_manifest.json
@@ -545,6 +545,19 @@
   },
   {
     "id": "firefox",
+    "version": "latest-beta",
+    "bitness": 64,
+    "url": "downloaded via mozdownload script, but a dummy directive is placed here so emsdk understands this Tool to be downloaded from the web",
+    "git_branch": "dummy field, to instruct emsdk to attempt to reinstall this tool even if it is installed, to check for new version",
+    "activated_cfg": "EMTEST_BROWSER=%actual_installation_dir%/firefox%.exe%",
+    "activated_env": "EMTEST_BROWSER=%actual_installation_dir%/firefox%.exe%",
+    "mac_activated_cfg": "EMTEST_BROWSER=%actual_installation_dir%/Contents/MacOS/firefox%.exe%",
+    "mac_activated_env": "EMTEST_BROWSER=%actual_installation_dir%/Contents/MacOS/firefox%.exe%",
+    "custom_install_script": "mozdownload_firefox",
+    "custom_is_installed_script": "is_firefox_installed"
+  },
+  {
+    "id": "firefox",
     "version": "nightly",
     "bitness": 64,
     "url": "downloaded via mozdownload script, but a dummy directive is placed here so emsdk understands this Tool to be downloaded from the web",