As mentioned elsewhere in this topic, unless the home interface is connected, firefox cannot access the files on its own (ie without the user driving file selection) when it is using portals. As such, on a firefox that supports portals (I think it is the one in stable now, but others can comment), you should be able to snap disconnect firefox:home and then just save your files in ~/snap/firefox/common. Firefox will only be able to access files in ~/snap/firefox (and a few things necessary to run, like input methods) from the user’s home directory “behind the scenes”. If you are worried about malicious sites and bugs in firefox, the worst firefox could be made to do with files outside of ~/snap/firefox is to trigger the portals file selector to display (but firefox itself can’t interact with the file selector).
That said, it seems like there is still a rough edge or two in the snap since I tried this and the snap seemed to make some assumptions about things. Once I was able to convince it to download to a directory in ~/snap/firefox/common, it worked mostly fine.