Even when the feature is disabled, ctrlTab.readPref is called, and calls uninit() which calls _init(false). I saw this showing up in a profile of restoring a session with 15 empty windows, and it was taking about 0.3% of the time. So the actual performance impact isn't really significant, but after reading the code out of curiosity I was tempted to clean this up, along with a few other code cleanups in the same file.
Created attachment 8850227 [details] [diff] [review] Patch
Pushed by email@example.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/bb6c2e991584 uninitialize ctrlTab only if it has been initialized before, r=dao.