Because all they're doing is then converting it into a message that triggers purging the session history in browser-content.js Instead, there should just be one observer (nsBrowserGlue.js seems to already do this and would probably do nicely) that broadcasts the async message once to every process, so it gets picked up in every browser-content.js. Then we also don't need to worry about removing the observer in the destroy() method, which doesn't always get called (see bug 1249439). It also looks like the starting time gets lost and we just purge all the history out of the browsers. Not sure if that can easily be fixed when fixing this, but it would be a nice bonus.
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