In Firefox changing the url hash by setting window.location.hash creates

a history entry. This behaviour is different from chrome et al so

instead we now use window.location.replace to avoid the extra entry.
Firefox will no longer return to #index_header when hitting the back

button after having clicked view diff.

Visited the diff viewer using View Diff in both Firefox and Chrome

before and after the fix. Chrome's behaviour is unchanged and Firefox

no longer has the additional unwanted history entry.