tests: timeslider_follow increase test timeout for firefox.

src/tests/frontend/specs/timeslider_follow.js

@@ -9,7 +9,7 @@ describe('timeslider follow', function () {
   // TODO needs test if content is also followed, when user a makes edits
   // while user b is in the timeslider
   it("content as it's added to timeslider", async function () {
-    this.timeout(10000);
+    this.timeout(20000);
     // send 6 revisions
     const revs = 6;
     const message = 'a\n\n\n\n\n\n\n\n\n\n';