Can you also try to run this code in the Browser Console:
> Cc["@mozilla.org/content-pref/service;1"].getService(Ci.nsIContentPrefService2).getBySubdomainAndName("icloud.com", "spellcheck.lang", null, {handleResult:console.log,handleCompletion:console.log})
and copy the output here.
You will first need to enable text entry in the Browser Console by setting the preference devtools.chrome.enabled to true in about:config
Bug 1523575 Comment 20 Edit History
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Can you also try to run this code in the Browser Console:
> Cc["@mozilla.org/content-pref/service;1"].getService(Ci.nsIContentPrefService2).getBySubdomainAndName("icloud.com", "spellcheck.lang", null, {handleResult:console.log,handleCompletion:console.log})
and copy the output here.
You will first need to enable text entry in the Browser Console by setting the preference `devtools.chrome.enabled` to true in about:config