I currently have 50 Flickr Sets, and wanted to sort them alphabetically. However there appears to be no function to do this. You can sort photos within a set but not the sets themselves on the Flickr page.
Sets within a Collection can be manually sorted, so I created a collection called ALL and have manually sorted these sets alphabetically. Took about 5 minutes but should be a one off exercise. Next time I create a set all I need do is add it to ALL collection abd move it to the relevant position.
To create this Collection and organize alphabetically go to the Flickr Organize screen and select "Collections and Sets" Tab, create new Collection and drag and drop the Sets into it making sure you drop them alphabetically. I did this by taking each Set in sequence as shown by default on the display, and as I dragged it into the column on the left under the ALL Collection I made sure I dropped it in the correct place alphabetically. If you miss you can position your mouse over it in the left column and drag it to correct place.