It's coming from here: http://hg.mozilla.org/mozilla-central/file/tip/browser/components/sessionstore/src/nsSessionStore.js#l2251 Something related to TabCandy and being in a weird state with hidden tabs... Apparently I had 0 unhidden tabs, and so we try to access aTabs[-1] :( I'll see if I can get some STR from a clean session. Interesting that the error says "aTabs[ex - 1] is undefined" instead of what's actually in the code, which is "aTabs[unhiddenTabs - 1]" Perhaps something from TM/JM merge?
I haven't seen this since (though tbh, I also haven't really used that feature much). I haven't seen any reports of it either, but we haven't hit b7 yet so I'll leave this open until this feature goes out to a wider audience.
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