Random memory leaks when tabs are closed

[DaveRandom]

Apparently at random (maybe 1/10 times?), when you close the tab (not not when you navigate away?) there is a memory leak which leaves references to elements on the now unloaded DOM in memory, and as a result the gc does not destroy all the JS objects correctly. Symptoms experienced so far are:

• "can’t access dead object" errors in console (see https://blog.mozilla.org/addons/2012/09/12/what-does-cant-access-dead-object-mean/)
• after a while, new tabs fail to initialise the application at all

There is a residual error with the shutdown procedure resulting from the recent refactoring in [cv-pls], for which I will commit a fix shortly, but I'm creating this issue and leaving it open as I am not 100% sure this will fix the problem, due to it's apparent randomness.