Session Restore should preserve normal browsing windows independently from Private Browsing windows

[mz33]

Firefox's "Open previous windows and tabs" session restore feature does not behave as expected when Private Browsing windows are involved.

Steps to reproduce:

1. Enable:
Settings → General → Startup → "Open previous windows and tabs"

2. Open a normal browsing window.

3. Open several tabs in the normal window.

4. Open a Private Browsing window.

5. Close the normal browsing window while the Private Browsing window is still open.

6. Close Firefox completely and restart it, or open a new normal browsing window.

Expected result:

The previously opened normal browsing tabs should be restored.

Private Browsing windows should not affect the session state of normal browsing windows, because Private Browsing does not participate in browsing history or persistent session data.

Actual result:

The previously opened normal browsing tabs are lost.

Firefox appears to treat the existence of Private Browsing windows as part of the overall browser session lifecycle, preventing the normal browsing session from being saved/restored.

This can cause accidental loss of a user's working tabs and is difficult to understand because there is no warning before the normal browsing session is discarded.

Suggested improvement:

Separate the session lifecycle of normal browsing windows from Private Browsing windows.

For example:

- Normal browsing windows should have their own restorable session state.
- Closing a normal browsing window should preserve its tabs for session restore.
- Private Browsing windows should remain non-persistent and should not prevent normal browsing session saving.

Rationale:

Users enable "Open previous windows and tabs" specifically to avoid manually restoring their working environment. However, the current behavior requires users to remember whether any Private Browsing windows are still open before closing Firefox, which defeats the purpose of automatic session restoration.

This behavior is especially confusing because Private Browsing is intentionally designed to be isolated from normal browsing data.

[Jon]

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