If you want to manipulate mozSettings with JavaScript, you can reference to Settings API on MDN (https://developer.mozilla.org/en-US/docs/Web/API/Settings_API). But if you want to manipulate it with C++, we can only reference to the codebase of Gecko. Now, let me show you some example.

1. Use C++ to Manipulate
mozSettings in Gecko
Mozilla Taiwan
Tommy Kuo [:KuoE0]
kuoe0@mozilla.com
cc-by-sa

2. mozSettings
Preference system in Gaia

3. Func1
Func2
Func1
Func4
Func3
mozSettings
get()
get()
set()
set()
get()

4. Func1
Func2
Func1
Func4
Func3
mozSettings
get()
get()
set()
set()
get()
What would happen if get and
set the same preference in the
same time?

5. mozSettings
critical section protected with a lock queue
Lock1 Lock2 Lock3 …
(active lock)

6. mozSettings
Lock1 Lock2 Lock3 Lock4Lock Queue
Func1 / Lock1
Func2 / Lock1
Func1 / Lock1
Func4 / Lock1
Func3 / Lock1
get()
get()
set()
set()
get()

7. Lock Queue Lock1 Lock2 Lock3 Lock4
(wait Lock1) (wait Lock2) (wait Lock3)

8. JavaScript
var lock = navigator.mozSettings.createLock();
var setting = lock.get('multiscreen.enabled');
setting.onsuccess = function () {
console.log('multiscreen.enabled: ' +
setting.result['multiscreen.enabled']);
}
setting.onerror = function () {
console.warn('An error occured: ' + setting.error);
}
More detail: https://developer.mozilla.org/en-US/docs/Web/API/Settings_API

9. JavaScript
var lock = navigator.mozSettings.createLock();
var setting = lock.get('multiscreen.enabled');
setting.onsuccess = function () {
console.log('multiscreen.enabled: ' +
setting.result['multiscreen.enabled']);
}
setting.onerror = function () {
console.warn('An error occured: ' + setting.error);
}
Rutern a DOMRequest
More detail: https://developer.mozilla.org/en-US/docs/Web/API/Settings_API

10. JavaScript
var lock = navigator.mozSettings.createLock();
var setting = lock.get('multiscreen.enabled');
setting.onsuccess = function () {
console.log('multiscreen.enabled: ' +
setting.result['multiscreen.enabled']);
}
setting.onerror = function () {
console.warn('An error occured: ' + setting.error);
}
Got result
More detail: https://developer.mozilla.org/en-US/docs/Web/API/Settings_API

11. JavaScript
var lock = navigator.mozSettings.createLock();
var setting = lock.get('multiscreen.enabled');
setting.onsuccess = function () {
console.log('multiscreen.enabled: ' +
setting.result['multiscreen.enabled']);
}
setting.onerror = function () {
console.warn('An error occured: ' + setting.error);
}
Got error
More detail: https://developer.mozilla.org/en-US/docs/Web/API/Settings_API

12. C++
#include "nsISettingsService.h"
nsCOMPtr<nsISettingsService> settings =
do_GetService("@mozilla.org/settingsService;1");
nsCOMPtr<nsISettingsServiceLock> settingsLock;
settings->CreateLock(nullptr,
getter_AddRefs(settingsLock));
nsRefPtr<GetBoolTask> callback = new GetBoolTask();
settingsLock->Get("multiscreen.enabled", callback);

13. #include "nsISettingsService.h"
nsCOMPtr<nsISettingsService> settings =
do_GetService("@mozilla.org/settingsService;1");
nsCOMPtr<nsISettingsServiceLock> settingsLock;
settings->CreateLock(nullptr,
getter_AddRefs(settingsLock));
nsRefPtr<GetBoolTask> callback = new GetBoolTask();
settingsLock->Get("multiscreen.enabled", callback);
C++
Get settings service

14. #include "nsISettingsService.h"
nsCOMPtr<nsISettingsService> settings =
do_GetService("@mozilla.org/settingsService;1");
nsCOMPtr<nsISettingsServiceLock> settingsLock;
settings->CreateLock(nullptr,
getter_AddRefs(settingsLock));
nsRefPtr<GetBoolTask> callback = new GetBoolTask();
settingsLock->Get("multiscreen.enabled", callback);
C++
Get settings lock

15. #include "nsISettingsService.h"
nsCOMPtr<nsISettingsService> settings =
do_GetService("@mozilla.org/settingsService;1");
nsCOMPtr<nsISettingsServiceLock> settingsLock;
settings->CreateLock(nullptr,
getter_AddRefs(settingsLock));
nsRefPtr<GetBoolTask> callback = new GetBoolTask();
settingsLock->Get("multiscreen.enabled", callback);
C++
Set callback

16. C++
#include "nsISettingsService.h"
nsCOMPtr<nsISettingsService> settings =
do_GetService("@mozilla.org/settingsService;1");
nsCOMPtr<nsISettingsServiceLock> settingsLock;
settings->CreateLock(nullptr,
getter_AddRefs(settingsLock));
nsRefPtr<GetBoolTask> callback = new GetBoolTask();
settingsLock->Get("multiscreen.enabled", callback);
What’s this?

17. nsISettingsServiceCallback
[scriptable, uuid(aad47850-2e87-11e2-81c1-0800200c9a66)]
interface nsISettingsServiceCallback : nsISupports
{
void handle(in DOMString aName, in jsval aResult);
void handleError(in DOMString aErrorMessage);
};
the callback object for settings request

18. nsISettingsServiceCallback
[scriptable, uuid(aad47850-2e87-11e2-81c1-0800200c9a66)]
interface nsISettingsServiceCallback : nsISupports
{
void handle(in DOMString aName, in jsval aResult);
void handleError(in DOMString aErrorMessage);
};
the callback object for settings request
onsuccess

19. nsISettingsServiceCallback
[scriptable, uuid(aad47850-2e87-11e2-81c1-0800200c9a66)]
interface nsISettingsServiceCallback : nsISupports
{
void handle(in DOMString aName, in jsval aResult);
void handleError(in DOMString aErrorMessage);
};
the callback object for settings request
onerror

20. GetBoolTask
class GetBoolTask final : public nsISettingsServiceCallback {
public:
NS_DECL_ISUPPORTS
NS_IMETHOD Handle(const nsAString& aName, JS::Handle<JS::Value> aResult)
{
PRINT("%s: %s", ToNewCString(aName), aResult.toBoolean() ? "True" :
"False");
return NS_OK;
}
NS_IMETHOD HandleError(const nsAString& aMsg)
{
PRINT("error: %s", aMsg);
return NS_OK;
}
protected:
~GetBoolTask() {}
};
inherit from nsISettingsServiceCallback

21. GetBoolTask
class GetBoolTask final : public nsISettingsServiceCallback {
public:
NS_DECL_ISUPPORTS
NS_IMETHOD Handle(const nsAString& aName, JS::Handle<JS::Value> aResult)
{
PRINT("%s: %s", ToNewCString(aName), aResult.toBoolean() ? "True" :
"False");
return NS_OK;
}
NS_IMETHOD HandleError(const nsAString& aMsg)
{
PRINT("error: %s", aMsg);
return NS_OK;
}
protected:
~GetBoolTask() {}
};
onsuccess
inherit from nsISettingsServiceCallback

22. GetBoolTask
class GetBoolTask final : public nsISettingsServiceCallback {
public:
NS_DECL_ISUPPORTS
NS_IMETHOD Handle(const nsAString& aName, JS::Handle<JS::Value> aResult)
{
PRINT("%s: %s", ToNewCString(aName), aResult.toBoolean() ? "True" :
"False");
return NS_OK;
}
NS_IMETHOD HandleError(const nsAString& aMsg)
{
PRINT("error: %s", aMsg);
return NS_OK;
}
protected:
~GetBoolTask() {}
};
inherit from nsISettingsServiceCallback
onerror

23. cc-by-sa
Thanks.