I finally switching from my own modified Freshrss with "hardcoded" QuickCollapse" button to the normal FreshRSS with the QuickCollapse Extension.
In order to be able to easily update, I checked out this Extension repo next to the FreshRSS folder and added the Extension as a symlink:

www-data@***:~/FreshRSS/extensions$ ls -la
total 16
drwxrwxr-x  2 www-data www-data 4096 Jan 11 12:13 .
drwxr-xr-x 14 www-data www-data 4096 Jan 11 12:00 ..
-rw-rw-r--  1 www-data www-data    6 Nov  9 16:19 .gitignore
-rw-rw-r--  1 www-data www-data  197 Nov  9 16:19 README.md
lrwxrwxrwx  1 www-data www-data   50 Jan 11 12:13 xExtension-QuickCollapse -> ../../FreshRSSExtensions/xExtension-QuickCollapse/

This seemd to work fine at first. The extension appeared in the config.
But it didn't work.

When analyzing the issue with the Browser Dev Tools i noticed that the javascript wasn't loaded.
When opening <domain>/freshrss/ext.php?f=xExtension-QuickCollapse%2Fstatic%2Fscript.js&t=js&1704970022 instead of the javascript file i only get:
File is not supported

I have now removed the symlink and just copied the folder over to the extensions folder …
Now it works fine.

It would be nice if this would be possible if it isn't too difficult to fix.

Btw: I think it would make sense to ignore anything in the extensions folder, not only stuff startig with "x". But I don't know if it is worth it's own issue in the FreshRSS repo.

Edit: This was with FreshRSS 1.22.1. After installing the extension I have now realized there was a new release.
I have now updated to 1.23.1 but not tested this issue again.