UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:51.0) Gecko/20100101 Firefox/51.0

Example URL:

Steps to reproduce the problem:
1. Load https://bug1295839.bmoattachments.org/attachment.cgi?id=8782242
2. Click the "TEST" button to open a popup window.
3. Reload the page loaded in step 1.
4. Click the "TEST" button again and observe that the already-opened window is reused for the load.
5. Close the popup.
6. Load https://bug1295839.bmoattachments.org/attachment.cgi?id=8784376
7. Click the "TEST" button to open a popup window.
8. Reload the page loaded in step 6.
9. Click the "TEST" button again and observe that a new popup is opened.

What is the expected behavior?
Step 9 and step 4 should have consistent behavior per spec and sanity.

What went wrong?
For some reason the sandbox attribute is making Chrome reuse a popup when it wouldn't reuse it otherwise.

Does it occur on multiple sites: N/A

Is it a problem with a plugin? N/A 

Did this work before? N/A 

Does this work in other browsers? Yes 

Chrome version: 54.0.2837.0 (Official Build) dev (64-bit)  Channel: n/a
OS Version: OS X 10.10
Flash Version: 

Gecko is inconsistent in the opposite direction here.  Safari is consistent between the two cases: never reuses the popup.