Cloned bug 1514073 due to reports of this affecting Mac too. Specifically bug 1514073 comment 12.

+++ This bug was initially created as a clone of Bug #1514073 +++

Firefox 66 nightly on windows10 and C:\WINDOWS\system32\Macromed\Flash\NPSWF64_32_0_0_101.dll

While investigating bug 1513966 I discovered that the Flash plugin can not save it settings anymore

9:33.11 INFO: No more inbound revisions, bisection finished.
9:33.11 INFO: Last good revision: ba708fde30b8f8d2ae4d1febfa2e88e2c1bf1cc4
9:33.11 INFO: First bad revision: a910482f4598931944910a357431b22f823578fb
9:33.11 INFO: Pushlog:
https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=ba708fde30b8f8d2ae4d1febfa2e88e2c1bf1cc4&tochange=a910482f4598931944910a357431b22f823578fb

STR:

1. load https://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager03.html
2. change for example the global storage setting to 10MB
3. switch the tab inside the flash settings area (example: Privacy settings) and switch back to the global storage tab
4. the changed setting from step 2 is back to it's original setting.

setting dom.ipc.plugins.sandbox-level.flash to "0" followed by a restart fixes the problem.

I can reproduce the problem and see that modifying a setting in the settings manager triggers a sandbox violation for file-write-unlink for the path below. We already allow file-read* file-write-create file-write-data for that directory in our Flash sandbox rules, but not file-write-unlink. After adding the file-write-unlink permission, the settings applet appears to work and the changes persist.

Sandbox: plugin-container deny(1) file-write-unlink /Users/username/Library/Preferences/Macromedia/Flash Player/macromedia.com/support/flashplayer/sys/settings.sol

Allow file-write-unlink in ~/Library/Preferences/Macromedia/Flash Player to allow changes made in the Flash Global Settings panel to persist.

https://hg.mozilla.org/mozilla-central/rev/71adb32bf578

Comment on attachment 9042621 [details]
Bug 1525625 - [Mac] Flash plugin can not save settings r?Alex_Gaynor

Beta/Release Uplift Approval Request

Feature/Bug causing the regression

Bug 1474375

User impact if declined

Changes made in the Flash global settings applet/pane do not take effect and do not persist. If a user needs to change one of the global settings, such as the global storage size, they will not be able to.

Is this code covered by automated tests?

No

Has the fix been verified in Nightly?

No

Needs manual test from QE?

Yes

If yes, steps to reproduce

See bug description.

List of other uplifts needed

None

Risk to taking this patch

Low

Why is the change risky/not risky? (and alternatives if risky)

The change only makes the Mac Flash NPAPI plugin sandbox slightly less restrictive by allowing removal of files in a particular Flash-specific directory. Unlikely to cause regressions.

String changes made/needed

None

Comment on attachment 9042621 [details]
Bug 1525625 - [Mac] Flash plugin can not save settings r?Alex_Gaynor

Looks low risk enough, let's uplift for beta 8.
Verification could be in either beta or nightly.

https://hg.mozilla.org/releases/mozilla-beta/rev/cea43c1728b9

I have reproduced this issue using Firefox 67.0a1 (2019.02.06) on macOS 10.13.
I can confirm this issue is fixed, I verified using Firefox 67.0a1 (latest nightly build) and 66.0b8 (build from Taskcluster) on macOS 10.13.