Tests: Run tests on both real Firefox ESRs

1. At the same time, there may be two supported versions of Firefox ESR. Run
   tests on both, installed locally.
2. Contrary to what we did on `main`, still run tests on Firefox 115 on
   BrowserStack - on `main`, we deleted it since we support only the versions
   supported upstream. In jQuery Migrate 3.x, we're testing on all versions
   matching ESR lines starting with Firefox 48, so for consistency let's keep
   Firefox 115 there as well.

Closes gh-582
Ref jquery/jquery#5547

(cherry picked from commit 41d038b)

Author: mgol

.github/workflows/browser-tests.yml

@@ -22,6 +22,12 @@ jobs:
           - NAME: "Node"
             BROWSER: "chrome"
             MIGRATE_VERSION: "esmodules"
+          - NAME: "Firefox ESR (new)"
+            BROWSER: "firefox"
+            MIGRATE_VERSION: "min"
+          - NAME: "Firefox ESR (old)"
+            BROWSER: "firefox"
+            MIGRATE_VERSION: "min"
     steps:
       - name: Checkout
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
@@ -39,6 +45,23 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-node-${{ env.NODE_VERSION }}-npm-lock-
 
+      - name: Set download URL for Firefox ESR (old)
+        run: |
+            echo "FIREFOX_SOURCE_URL=https://download.mozilla.org/?product=firefox-esr-latest-ssl&lang=en-US&os=linux64" >> "$GITHUB_ENV"
+        if: contains(matrix.NAME, 'Firefox ESR (old)')
+
+      - name: Set download URL for Firefox ESR (new)
+        run: |
+            echo "FIREFOX_SOURCE_URL=https://download.mozilla.org/?product=firefox-esr-next-latest-ssl&lang=en-US&os=linux64" >> "$GITHUB_ENV"
+        if: contains(matrix.NAME, 'Firefox ESR (new)')
+
+      - name: Install Firefox ESR
+        run: |
+            wget --no-verbose $FIREFOX_SOURCE_URL -O - | tar -jx -C ${HOME}
+            echo "PATH=${HOME}/firefox:$PATH" >> "$GITHUB_ENV"
+            echo "FIREFOX_BIN=${HOME}/firefox/firefox" >> "$GITHUB_ENV"
+        if: contains(matrix.NAME, 'Firefox ESR')
+
       - name: Install dependencies
         run: npm install