There is a known issue that will cause Firefox to crash when you send files using an encrypted file transfer service while FPN is on. This is a known issue that will be fixed in an upcoming release.
In the meantime, turning off FPN while you’re doing the file transfer should prevent Firefox from crashing:
- Click the Private Network button
on the toolbar to open the options panel.
- Click the or button (or toggle the switch next to Firefox Private Network. Blue means it’s on).