lost focus(focus ring) on a link after browser back

[alice0775]

Steps to reproduce:

1.Install 'Dark Background and Light Text' extension from https://addons.mozilla.org/ja/firefox/addon/dark-background-light-text/
2.Open web page ( e.g. https://ftp.mozilla.org/pub/firefox/nightly/ )
3.Press [Tab] key several times to focus a link ( e.g. 2005/ ), so that a focus ring will appear around "2005/".
4.Press [Enter] key to open it
5.Press [alt+left arrow] key to browser back

Actual results:
The focus ring would not appear around a link "2005/".

Expected results:
A focus ring should appear(restore) around a link "2005/".

Using 'Simple CSS' instead of 'Stylesheet processor' will fix this issue.