EDIT: Feedbro 3.41.7 now supports X-Frame-Options modification so view mode 6 should work for all sites.

Note that you have to set "Trackers" setting to "Only in Private Windows" in about:preferences#privacy if you want to load Reddit feeds. This is because Firefox has an internal "tracker list" where Reddit is blacklisted. So Firefox blocks all XMLHttpRequests to Reddit if you have Trackers setting set to Always.

Other than that I guess your comment is valid especially for view mode 6 (embedded iframe view). Some sites send HTTP Response headers that prevent embedding their site in an iframe. We'll check if it's possible to get around this without introducing performance penalties or other side effects.

1. Yes it's intentional that items marked as read aren't hidden right away. This gives the user the possibility to "undo" the mark as read operation.

2. That's true. Note that you can collapse by pressing Enter but currently "closing by clicking" isn't supported. This is because closing isn't really needed and a careless attempt to click a hyperlink in the article could result in collapsing the article which could infuriate some users (we tested this :)).

3. Custom CSS support would be easy to implement but the CSS hasn't been very stable (we have changed it a lot along the way) which could easily lead to breaking custom CSS configs and an avalanche of support requests and complaints. So it hasn't been implemented so far. We are open to all UI improvement ideas though.

Feedbro has built-in partial feed->full text feed engine. Right click the feed in the feed tree, select Properties and then experiment with the "Feed Entry Content" setting (change it + click Preview). Then Save. New articles loaded after that will use the new setting.