Support Multiprocess Firefox (e10)

A bug is open on Mozilla to support Firefox [e10](https://wiki.mozilla.org/Electrolysis) (multiprocess Firefox): [bug 1042965](https://bugzilla.mozilla.org/show_bug.cgi?id=1042965)

Nicholas Nethercote says:

> When you mouse over a text box, the small "edit" button is supposed to appear, and it disappears again when you move the cursor away.
> With e10s enabled, this doesn't happen. However, if I switch to another tab and then switch back, it _does_ happen. And you can click on it and edit the text in a separate text editor, as expected.
> So the extension is usable, but the behaviour is confusing and annoying. (In fact, it's possible that this behaviour has been present since the bug was filed, but nobody realised because it's so surprising.)