A few times now, Firefox has been refusing to open Dolphin (or anything else for that matter) when, for instance, clicking on a button to browse for files on any file upload form on a web page. I also have the same issue when right clicking a link, save link as. Absolutely nothing happens. It just silently fails to do anything. Just today I was viewing a pdf in Firefox and the print button would do nothing at all. I have to open chromium to accomplish any of those tasks successfully. This issue seems random to me, as it just happens for a while, then starts working again at some random point.

I've made sure Dolphin is in first in the list in System Settings > Applications > File Associations > inode > directory, in the Application Preference Order (the only other app there is Gwenview). Dolphin was already at the top so I moved Gwenview to the top and applied, then moved Dolphin back to the top and applied. I did reboot after doing all that.

I couldn't find much else that would specifically relate to my issue when searching for a solution.

Any help or clues would be very appreciated.

Edit:
I have it set to ask me where to save files when downloading, and when downloading a file, the save dialog doesn't come up. Again, just a silent fail.

I did just completely purge firefox and remove my entire ~/.mozilla directory, reinstalled firefox, and it still can't seem to interface with plasma in any way, to the point of having to forcefully kill firefox to get it to close if I even attempt to print a web page. chromium is still working, but I definitely don't want to be using chromium.

I haven't been able to find any possible solution yet.

Looks like you need to have have xdg-desktop-portal and xdg-desktop-portal-kde installed. On my system xdg-desktop-portal-kde was already installed but xdg-desktop-portal was not.