1.9.1 beta 2 checkins

There are a lot of things queuing up to check in for the 1.9.1 beta 2 freeze. We want to make sure that there's adequate test coverage for everything, so we know what to back out when tests fail. Please sign up below for a chance to push.

Name	Description/Bug#
bent	js exceptions off main thread/web worker spec changes
smaug	bug 454108
roc	Trivial bug 462972, nontrivial image snapping changes bug 458487, low-risk column changes bug 462307
mak77	bug 445737, bug 418643
dietrich	bug 419792, bug 457473, bug 407110
gavin	bug 235853 PAC bug, bug 461803 elementFromPoint changes, bug 17612 view-source link browsing front-end patch (very safe)
smontagu	bug 428317 Indic ligatures on OSX
rob_strong	bug 324121 Extensions with available updates for the next version of the app should not be listed in app update extension will be disabled warning