You can create a folder for each feed. Pick the corresponding folder when subscribing to a feed.

I imported my feeds from an OPML file, and this is how Thunderbird organized them for me.

I have about 150+ feeds in that file, so I hope I do not have to re-add them by hand to get them in a folder... Also, if they were not in a folder yet, I would expect the manage subscriptions screen to show them as one long list, but it shows the folders there.

At the moment I only see the folder name there, but I cannot open up that folder to see the individual feeds.

May be you could post a screenshot of what you're seeing, and explain in more detail what you're trying to achieve. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

I have added 2 screenshots. The small screenshot shows how the feeds look in my account view: only folder titles and no "open" option. The big screenshot shows how the feeds look in the "manage feed subscriptions" view: you can see the same "(Board) Games & Gaming" folder, but here you can open it to show the individual feeds within the folder.

I see what you mean. But I'm afraid there is no easy solution to your problem. I may be wrong, but I believe this is just the way TB organizes feeds, which isn't very sophisticated. I'd assume you'd then see posts from all feeds inside the "(Board) Games & Gaming" folder mixed up.

As previously stated, I'm organizing my feeds in separate sub-folders, one for each feed. This way the folder tree can be expanded and collapsed, and I only see posts which belong to one particular feed.

What you may want to try is to manually create sub-folders, and then drag and drop each feed to the corresponding sub-folder. Still quite an effort, but at least you wouldn't have to manually re-create each feed. I haven't tried that though.