After testing a ttrss demo mobile frontend on https://icnerd.com/2013/05/21/tiny-tiny-rss-demo-site/ I may have finally found a solution for my request - using tt-rss from the Firefox sidebar.

When loading the mobile webapp in the sidebar - what do I have to modify so that clicking on categories or feeds does not make them load in the main window but in the sidebar?

And could you add a button on the bottom of the list of articles to load more articles?

And, third, there should also be a "mark all articles as read" button!
Thank you for your help!