You can merge all bookmarks of a widget into another widget.
Instructions
Click the Widget menu button in the top-right corner of the Bookmarks widget you want to merge with another widget (three dots).
In the Widget menu, select the option Copy, merge & move.
Select the operation you want to perform, which are to copy, merge or move the widget.
Now, select the destination widget. First, you choose the page, then the widget you want to send your bookmarks to.
Click Merge (or Copy or Move) to confirm.
If you merge a Bookmarks widget with groups to a Bookmarks widget without groups, we will create a separate tab called New Group in the destination widget. This is where you'll find the original bookmarks of the original widget. The other tab names will be respected.
Warning
When you move all bookmarks from a widget to another one, the source Bookmarks Widget will disappear. This can't be undone.