After comparing this to the default XML bookmarks file in the app, I found that adding the following to each XML attribute within the 'folder=' created a clickable group of links:
group="true"Just add this to any folder to create a clickable group again. For example, to create a folder of news sites that you have named "News" into a clickable group, make the folder entry look like this:
<folder name="News" group="true">[Editor's note: Although it's been discussed here before, as a quick reminder, you can create groups in Chimera by first opening new tabs with each site you want as a member of the group, then (on any tab) selecting Bookmarks -> Add Page to Bookmarks, and checking the "Bookmark all tabs" box.]

