Make the `H` and `L` commands more reliable

Make the `H` and `L` commands more reliable

It seems like using `window.BrowserBack()` and `window.BrowserForward()` are the
most reliable ways of navigating the history, rather than having to wait for
`SessionStore.getSessionHistory()` to finish and then going to a certain history
index. This commit optimizes the case where the count is 1 to use the mentioned
functions instead. Some extensions also override those, so calling them results
in better interoperability.

(Hopefully/Likely) fixes #687.

To keep the code changes simple, the support for Firefox < 43 was dropped.
That's fine, since Firefox 44 has already been released.