When you download and click on a file without the program to use it installed, the KDE portal opens up, and the only option is to go to the discover store to get the required program since you dont have anything else that's usable. However, the new program does not show up in this window, and the window to choose the associated program cannot be closed. So, to get it to go away, I chose to associate .pptx with the dolphin browser temporarily so I could choose the correct program afterwards. After going into the firefox settings (General>applications) and setting the powerpoint files to open with ONLYOFFICE, it still ignores my changes and opens up the dolphin browser every time. I set the normal linux file association to ONLYOFFICE, so clicking on the file outside of firefox opens the correct program. I tried uninstalling firefox and deleting "all" associated files through the discover store, and still had the same problem upon reinstalling. I can clearly see there are some remaining Mozilla files on my drive despite the fact I said to delete all of the associated files, but for some reason I do not have permission to delete them, no matter what I try. When downloading and clicking a .pptx file through chromium, the correct application loads right away, so this is clearly a problem with only firefox not updating its file associations despite me repeatedly trying to do so through the system settings.