I don't know whether the comments I posted last August are relevant to your scenario, since the xdg-desktop-portal issue usually affects ALL use of the file chooser:

https://support.mozilla.org/questions/1386312

Maybe it's some kind of file permission problem??

This is interesting and it would be helpful if you provide steps and screenshots to replicate your issue.

Yes so for this example I will be using the snap version of firefox.

I mount my network drive through gnome and load up my web-based email client. Clicking the attach button allows me to navigate to a simple html file and select it (Shot 1). When I click "Open", nothing happens and I just get (Shot 2).

If I get the direct path to the file on my machine I can confirm I have access to it. (Shot 3)

Strangely if I use the .deb version directly from Mozilla I cannot see the network mount using the GTK3 file picker. If I switch to the KDE selector I can, but I get a clear message that I don't have permission.

I am happy to get more information, I will admit I'm a bit lost trying these different options out.

Note that Snap has limitations when it comes to accessing local files. Files in the Home directory should be Okay, by files located elsewhere might not be accessible.

yes I had originally suspected that, but it is actually with the .deb install from the Mozilla PPA that I can't even see the network mount via the GTK file picker. All very strange behavior. Does this work for anyone? I know Ubuntu hasn't made things the easiest with all the snap nonsense.