fixup! Bug 3455: Add DomainIsolator, for isolating circuit by domain.

Bug 42031: Fixed some requests failing because of a type error

Some requests (e.g., uBlock Origin filter updates) failed because they
the TorDomainIsolator tried to access the browser from their channels.

toolkit/components/tor-launcher/TorDomainIsolator.sys.mjs

@@ -453,7 +453,7 @@ class TorDomainIsolatorImpl {
    */
   #getBrowserForChannel(channel) {
     const browsers =
-      channel.loadInfo.browsingContext?.topChromeWindow?.gBrowser.browsers;
+      channel.loadInfo.browsingContext?.topChromeWindow?.gBrowser?.browsers;
     if (!browsers || !channel.loadInfo.browsingContext?.browserId) {
       return null;
     }