This seems to be a GTK problem, that is already fixed and merged into the codebase, but not released for ubuntu yet.
I found another tipp on that though, that works for me:
When you have the save dialog open, click the filename field (only focus needed) and then save it normally.
As SimonEritsch stated, Mozilla seems to be aware of the issue and I'd imagine a fix will come soon. In the meantime, I highly recommend installing Firefox via apt. Canonical (the company that develops Ubuntu) is really pushing snap, so it takes a few extra steps, but you can find those steps here.
This seems to be a GTK problem, that is already fixed and merged into the codebase, but not released for ubuntu yet.
I found another tipp on that though, that works for me: When you have the save dialog open, click the filename field (only focus needed) and then save it normally.
Found that here: https://forum.vivaldi.net/post/579271
See OCDev's comment, my answer is incorrect
As SimonEritsch stated, Mozilla seems to be aware of the issue and I'd imagine a fix will come soon. In the meantime, I highly recommend installing Firefox via apt. Canonical (the company that develops Ubuntu) is really pushing snap, so it takes a few extra steps, but you can find those steps here.