Upload backup fails with "unsupported file format" under Firefox for Windows

[ThermoMan]

Describe the issue you are experiencing

I have the same issue as #3639

I have a fresh backup from

Installation method Home Assistant Container
Core 2025.7.3
Frontend 20250702.3

I am attempting to restore to a new install of HAOS made today from
https://github.com/home-assistant/operating-system/releases/download/16.3/haos_ova-16.3.vdi.zip

I get the error message

Unsupported file format

Please choose a Home Assistant backup file (.tar)

I am on Waterfox 6.6.7 (current version)

the backup is named automatic_backup_2025_7_3.tar and opens without error in WinRAR.

What type of installation are you running?

Home Assistant OS

Which operating system are you running on?

Home Assistant Operating System

Steps to reproduce the issue

1. Visit http://homeassistant.local:8123/onboarding.html?page=restore_backup
2. Select file automatic_backup_2025_7_3.tar
3. That's it
   ...

Anything in the Supervisor logs that might be useful for us?

No logs

System information

the thing is not set up so no sys info is available.

It's in a VirtualBox VM with 4GB RAM and 2 CPUs set up according to https://www.home-assistant.io/installation/alternative

Supervisor diagnostics

No response

Additional information

No response

[ThermoMan]

I did find a work around using Brave. Why the problems with Firefox?

Brave 1.85.118 (Official Build) (64-bit)