The certificate error UI was modernized in Firefox 44:
https://bugzilla.mozilla.org/show_bug.cgi?id=1207107

Instead of sending keys directly to a specific window, activate the
window, then send the keys to the active window.

This is work-around for an xdotool bug[0], which broke sending synthetic
keys, such as Ctrl+F, to a specific window.

The downside of this approach is that the user could break the test if
they switched to another window while the test is running. This is not a
big deal, because one should run automated tests in an isolated
environment (for example, under Xvfb) anyway.

This commit should be reverted once the xdotool bug is fixed.

[0] jordansissel/xdotool#52

Use Escape followed by Shift+Tab to select buttons after searching for
their label.

Shift+Tab alone, which is was what used previously, happened to work
only by accident, and broke in Firefox 46.

Thanks to Sean Whitton for the bug report.

The previous approach of enabling the extension in extensions.ini was
unnecessarily complicated and stopped working with Firefox 48.