chore: compile to Firefox >= 53

it's the min version in quite a few Babel plugins/transforms:
https://github.com/babel/babel/blob/master/packages/babel-preset-env/data/plugins.json

.babelrc.js

@@ -5,12 +5,6 @@ module.exports = {
       ...process.env.BABEL_ENV !== 'test' && {
         modules: false,
       },
-      exclude: [
-        // for Firefox 52, see babel/babel#8204
-        'transform-regenerator',
-        // all features of this transform are natively supported since Chrome 49, Firefox 52
-        'transform-parameters',
-      ],
       useBuiltIns: false,
     }],
   ],

.browserslistrc

@@ -1,2 +1,2 @@
 Chrome >= 55
-Firefox >= 52
+Firefox >= 53