[Affected Versions]:

• Firefox Beta 88.0b2 (Build ID: 20210323190052)
• Firefox Nightly 89.0a1 (Build ID: 20210324084848)

[Affected Platforms]:

• Windows 10
• Linux Mint 20
• macOS 10.15

[Prerequisites]:

• Have Firefox open.
• Have logins saved.

[Steps to reproduce]:

1. Navigate to “about:logins”.
2. Click the meatball menu button (“...”).
3. Choose “Export Logins” and complete the export process.
4. Open the CSV file and delete the timestamps.
5. Return to Firefox and open the meatball menu again.
6. Choose the “Import from a File..." option.
7. Select the edited CSV file and import it.
8. Observe the “Import Complete” modal.
9. Observe the dates of the login entries.

[Expected result]:

• The modal only detects duplicate logins.
• The existing entries are not updated.

[Actual result]:

• The modal detects that logins have been updated.
• The existing logins are updated to no longer have dates.

[Notes]:

• Attached is a recording of the issue.

i'm not sure what the expected result would be in this case. The logins should be updated, but at that point, the timestamps are no longer accurate.
We could use new timestamps from the time of import perhaps? Or try and get the last-modified time from the file itself? Or just leave the possibly-inacurate timestamps from the initial import as-is.