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 in jquerygh-5547, still run tests on Firefox 115 on
   BrowserStack - on `main`, we deleted it since we support only the versions
   supported upstream. In jQuery 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 jquerygh-5548
Ref jquerygh-5547

Author: mgol

.github/workflows/node.js.yml

@@ -37,7 +37,10 @@ jobs:
           - NAME: "Chrome"
             NODE_VERSION: "20.x"
             NPM_SCRIPT: "test:amd"
-          - NAME: "Firefox ESR"
+          - NAME: "Firefox ESR (new)"
+            NODE_VERSION: "20.x"
+            NPM_SCRIPT: "test:firefox"
+          - NAME: "Firefox ESR (old)"
             NODE_VERSION: "20.x"
             NPM_SCRIPT: "test:firefox"
     steps:
@@ -57,10 +60,21 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-node-${{ matrix.NODE_VERSION }}-npm-lock-
 
-      - name: Install firefox ESR
+      - 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: |
-          export FIREFOX_SOURCE_URL='https://download.mozilla.org/?product=firefox-esr-latest-ssl&lang=en-US&os=linux64'
           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
@@ -71,10 +85,7 @@ jobs:
         if: contains(matrix.NPM_SCRIPT, 'lint')
 
       - name: Run tests
-        run: |
-          export PATH=${HOME}/firefox:$PATH
-          export FIREFOX_BIN=${HOME}/firefox/firefox
-          npm run ${{ matrix.NPM_SCRIPT }}
+        run: npm run ${{ matrix.NPM_SCRIPT }}
 
   safari:
     runs-on: macos-latest