My personal reason to use the snap is because it limits access to the home directory. So I can disconnect my home directory, camera and microphone and have a second layer of confidence that my browser won't leak any personal data.
That and it avoids Firefox leaking config files in my home directory.
If I want to upload a file, I simply move the file into a Downloads folder in it's SNAP home directory. This way, Im in control of what the browser can access.
You would need to copy once (or `rsync`) from `~/.mozilla/firefox/` to `~/snap/firefox/current/.mozilla/firefox/`.
The alternative, and likely why no one has applied a great deal of pressure to that workflow, is to use the Firefox Sync account they've been pushing so hard