Build: Test on Firefox 140

This continues the strategy of testing on Firefox versions with major version
parts matching an ESR release.

Author: mgol

.github/workflows/browserstack.yml

@@ -40,6 +40,7 @@ jobs:
           - 'Edge_18'
           - 'Firefox_latest'
           - 'Firefox_latest-1'
+          - 'Firefox_140'
           - 'Firefox_128'
           - 'Firefox_115'
           - 'Firefox_102'