Fix regression that disabled the "Open With Different Viewer" button on the fallback bar

Clicking the "Open With Different Viewer" button on the fallback bar fails, because we no longer provide a callback to `FirefoxCom.request` (see: https://github.com/mozilla/pdf.js/blob/master/web/firefoxcom.js#L51).

Note that this could equally well be solved by just removing the `url` parameter, since it doesn't appear to be used anywhere in the code, but I wasn't sure if there was a reason for including this. I'll be happy to update the PR with this change instead!

This regressed in PR #4077.