FF 140.9.1 ESR (32b), Win10x64
In the settings → Applications → PDF - I set the default to “Save file”, exit FF.
I open it again: my “Save file” is reset to “Use default application”. The setting “Save file” is not saved!
Moreover, on all other file types, “Save file” is selected and all settings are successfully saved. Only with PDF there was some trouble. There are no any plugins for PDF. In safe mode the problem is the same, and not a single plugin works there at all.
btw, “Always ask” can be checked and it is saved, but “Save file” is always reset after a restart to “Use default application”