This has been annyoing me for years now. I use firefox as Browsre in KDE Plasma.
Firefox consistently uses wrong applications to open files. I can configure them in firefox settings dialog; say "pdf -> okular", "xls->libreoffice". I can, but it just has no effect.
Assuming the last opened file from firefox used 'libreoffice', the next time any download will just suggest to use "libreoffice". No matter its type. Assuming it was a PDF, I silently curse, and chose "okular" from the dropdown. Great. Now firefox just assumes I want to open all the following files in okular.
I have this behaviour on all my systems, FreeBSD 12/13, FF up to date.
I can confirm that this is also true in GTK based
DEs as well. It appears that FF will only use, or
offer to use applications chosen by the DE for
"preferred" applications to deal with content.
While FF indicated you can "make this change permanent"
in FF "preferences". It has little, if any effect.
As a result, I'm forced to right-click > save-as
in FF, to ever open content with my *chosen* application.
Please, provide clear steps to reproduce in a fresh profile. I don't use this feature, don't use any DE, don't use ocular/libreoffice and can't reproduce.