We have configured Firefox to open PDF attachments immediately in the browser. Unfortunately, this causes that when you click on the download button of an attachment, it still opens in a new tab (and additionally downloads in download mode). Unfortunately I can't say if this is related to Firefox 115.3esr or to SOGo 5.9, as both were released "at the same time". The request and response headers (Content-Type and Content-Disposition) look correct from my point of view.

Maybe it can be fixed by passing "Content-Transfer-Encoding: binary" or changing to the unknown Content-Type "application/force-download" (potentially dangerous). But it seems to be easier to add the HTML attribute "download" to the link (https://www.w3schools.com/tags/att_a_download.asp). I hope this can solve the problem.