feat: drop support for older Firefox versions

BREAKING CHANGE: Firefox < 57 is not supported anymore.

.babelrc

@@ -3,8 +3,7 @@
     [
       "env",
       {
-        "targets": {"chrome": 55, "firefox": 53, "opera": 42},
-        "exclude": ["transform-regenerator"]
+        "targets": {"chrome": 55, "firefox": 57, "opera": 42}
       }
     ]
   ],

package.json

@@ -17,17 +17,15 @@
     "exec": "yarn run build && yarn run start",
     "exec:prod": "yarn run build:prod && yarn run start",
     "zip": "web-ext build -s dist/ -a artifacts/",
-    "start": "web-ext run --bc --url 'about:debugging' --url 'about:addons' -s dist/",
-    "inspect": "webpack --profile --json > report.json; webpack-bundle-analyzer report.json --mode static && sleep 6 && rm report.{json,html}",
+    "start":
+      "web-ext run --bc --url 'about:debugging' --url 'about:addons' -s dist/",
+    "inspect":
+      "webpack --profile --json > report.json; webpack-bundle-analyzer report.json --mode static && sleep 6 && rm report.{json,html}",
     "update": "ncu --upgrade --upgradeAll && yarn",
     "push": "git push --follow-tags origin master",
     "release": "standard-version && yarn run push"
   },
-  "browserslist": [
-    "Chrome >= 55",
-    "Firefox >= 53",
-    "Opera >= 42"
-  ],
+  "browserslist": ["Chrome >= 55", "Firefox >= 57", "Opera >= 42"],
   "dependencies": {
     "@material/list": "^0.27.0",
     "@material/ripple": "^0.27.0",

src/manifest.json

@@ -10,7 +10,7 @@
   "applications": {
     "gecko": {
       "id": "{ce9f4b1f-24b8-4e9a-9051-b9e472b1b2f2}",
-      "strict_min_version": "53.0"
+      "strict_min_version": "57.0"
     }
   },
   "minimum_chrome_version": "55.0",

src/storage/storage.js

@@ -2,7 +2,6 @@ import browser from 'webextension-polyfill';
 
 import migrate from 'storage-versions';
 
-// Firefox <= 52
 let syncArea;
 async function getSupportedArea(requestedArea) {
   if (typeof syncArea === 'undefined') {