Thank you for your feedback.

You said:
1. One serious improvement should be that when you click on a headline the article will appear in a Firefox tab. Currently you have to click on the headline and then click on the article that appears the the bottom of the sidebar to get it to open in a tab.

2. A minor problem is that you can't initially do anything with a folder after you create it. You can only change its position in the list and put feeds in the folder if there's already at least one feed in the folder - which can be done in the subscribing step.

About 1.
I not sure to understood well.
There are many way to use Drop Feeds.
a. you can open feed in a Firefox tab and the click on a article headline and the article will be opened in a Firefox tab
b. you can display feed (list of article headlines) bottom feeds treeview in the side bar and click on a article headline and the article will be opened in a Firefox tab
c. you can display feed (list of article headlines) bottom feeds treeview in the side bar and click on a article headline and the article will be display in the side bar bottom the feed (list of article headlines), and then click on the article headline to open it in a Firefox tab.
d. different mix of options above...

Looks in Settings/Items and/or Settings/Content area to change displaying articles and feeds options

About 2.
I will look what I can do

You can create an issue on https://github.com/dauphine-dev/drop-feeds, then you can add a picture to see me what you want exactly about 1.

Edit:
In Drop Feeds 0.9.4 folder drag and drop has be improved, now it should be work