(another bmo side port)

Now that I've looked into the Bugzilla code (https://bugzilla.mozilla.org/extensions/InlineHistory/web/inline-history.js) my suspicion in comment 8 seems correct:

>          var itemHtml =  '<div class="ih_history_item ' + containerClass + '" '
>                          + 'id="h' + i + '">'
>                          + item[3]
>                          + '<div class="ih_history_change">' + item[2] + '</div>'
>                          + '</div>';
>
>          if (ih_activity_sort_order == 'oldest_to_newest') {
>            currentDiv.innerHTML = currentDiv.innerHTML + itemHtml;
>          } else {
>            currentDiv.innerHTML = itemHtml + currentDiv.innerHTML;
>          }

This is being done for each single history item and in case of bug 705294 currentDiv is always the same element (there is only one comment so all history items are being inserted into the same place). Each time an item is being added the HTML code for the entire block (including all the previously added items) has to be reparsed. The most trivial way to avoid this issue would be replacing the code above by one that doesn't touch already existing elements:

>          var item = document.createElement("div");
>          item.className = "ih_history_item " + containerClass;
>          item.id = "h' + i;
>          item.innerHTML = item[3] + '<div class="ih_history_change">' + item[2] + '</div>';
>
>          if (ih_activity_sort_order == 'oldest_to_newest') {
>            currentDiv.appendChild(item);
>          } else {
>            currentDiv.insertChild(item, currentDiv.firstChild);
>          }