Firefox cannot use symlinked directories for reading or writing

My ~/Downloads directory is symlinked to a slower disk. Firefox is unable to save any downloads; they show up as failed. This also happens using 'Save image as ...'

Further, if I'm uploading a file to a website, it can't be from a symlinked directory (in my case, ~/tmp).

Other context: this is a fresh Kubuntu 22.10 install:
Description: Ubuntu 22.10
Release: 22.10

Firefox is version 112.0.1+build1-0ubuntu0.22.10.1~mt1 from the Mozilla PPA (not Beta). I didn't have this problem using what I thought was the same lineage of package on the previous desktop.

I tried this using a pristine profile, and the problem still remained.