Support disabling `suspendTabsUntilReady` in Firefox

The value of `suspendTabsUntilReady` was disregarded in Firefox and
uBO defaulted to always defer tab loading until it was ready.

This commit allows to disable the deferring of tab loading in
Firefox. The new valid values for `suspendTabsUntilReady` are:
- `unset`: leave it to the platform to pick the optimal
  behavior (default)
- `no`: do no suspend tab loading at launch time
- `yes`: suspend tab loading at launch time

src/js/background.js

@@ -53,7 +53,7 @@ const µBlock = (function() { // jshint ignore:line
         requestJournalProcessPeriod: 1000,
         selfieAfter: 11,
         strictBlockingBypassDuration: 120,
-        suspendTabsUntilReady: false,
+        suspendTabsUntilReady: 'unset',
         userResourcesLocation: 'unset'
     };
 
@@ -104,6 +104,11 @@ const µBlock = (function() { // jshint ignore:line
                         if ( out.hasOwnProperty(k) ) { out[k] = o[k]; }
                     }
                     self.log.verbosity = out.consoleLogLevel;
+                    if ( typeof out.suspendTabsUntilReady === 'boolean' ) {
+                        out.suspendTabsUntilReady = out.suspendTabsUntilReady
+                            ? 'yes'
+                            : 'unset';
+                    }
                 }
             }
             catch(ex) {

src/js/storage.js

@@ -110,6 +110,12 @@
                     this.hiddenSettings[key] = hs[key];
                 }
             }
+            if ( typeof this.hiddenSettings.suspendTabsUntilReady === 'boolean' ) {
+                this.hiddenSettings.suspendTabsUntilReady =
+                    this.hiddenSettings.suspendTabsUntilReady
+                        ? 'yes'
+                        : 'unset';
+            }
         }
         if ( vAPI.localStorage.getItem('immediateHiddenSettings') === null ) {
             this.saveImmediateHiddenSettings();

src/js/traffic.js

@@ -1002,8 +1002,10 @@ return {
         if (
             vAPI.net.onBeforeReady instanceof Object &&
             (
-                vAPI.net.onBeforeReady.experimental !== true ||
-                µBlock.hiddenSettings.suspendTabsUntilReady
+                vAPI.net.onBeforeReady.experimental !== true &&
+                µBlock.hiddenSettings.suspendTabsUntilReady !== 'no' ||
+                vAPI.net.onBeforeReady.experimental &&
+                µBlock.hiddenSettings.suspendTabsUntilReady === 'yes'
             )
         ) {
             vAPI.net.onBeforeReady.start();