Right now we only see an assertion in the log but we don't know what pref we want to save. 19:19:14 INFO - [Child 879] WARNING: NS_ENSURE_TRUE(XRE_GetProcessType() == GeckoProcessType_Default) failed: file ../../../../gecko/modules/libpref/src/Preferences.cpp, line 1485 19:19:14 INFO - [Child 879] ###!!! ASSERTION: cannot save prefs from content process: 'Error', file ../../../../gecko/modules/libpref/src/Preferences.cpp, line 618 It would be great to print the pref name as well.

Reuben, something you could take?

Here's how it looks like: [Child 34370] WARNING: ENSURE_MAIN_PROCESS failed. Cannot SetInt from content process: dom.foo.bar: file /Users/Reuben/Development/b2g-inbound/modules/libpref/src/Preferences.cpp, line 1493 [Child 34844] ###!!! ASSERTION: ENSURE_MAIN_PROCESS failed. Cannot SetIntPref from content process: dom.foo.bar: 'Error', file /Users/Reuben/Development/b2g-inbound/modules/libpref/src/nsPrefBranch.cpp, line 205

Comment on attachment 832335 [details] [diff] [review] Include pref name in the error message for operations that cannot happen in content processes PR_smprintf allocs, and you're never freeing the buffer. Also, the string formatting isn't guarded by a debug-only block, which is what I think you want here, right?

You may want nsPrintfCString

You are of course correct on both comments.

Thanks! https://hg.mozilla.org/integration/mozilla-inbound/rev/1687f0278bdd