Some Admin pages have elements shifted of screen in Commons Roots theme

This is caused by Firefox improperly rendering the .block float: left; style beginning on line 1013 of the fusion_core styles.css stylesheet as applied to breadcrumbs block.

The float in this:

.block {
  float: left; /* LTR */
  margin-bottom: 1.5em;
  width: 100%;
}

improperly displays this:

<div id="breadcrumbs" class="breadcrumbs block">

which then interferes with the display of the .block items that follow. One fix is to remove the float from the breadcrumbs div on the admin page. If you're using the (default) admin module with toolbar, you can add this to your CSS:

body.admin-nw div#breadcrumbs.block,
body.admin-vertical div#breadcrumbs.block,
body.admin-df div#breadcrumbs.block {
  float: none;
}

If you're not using the admin toolbar, you'll have to find another way to target the breadcrumbs on an admin page, possibly by changing an admin page template to include an 'admin' class which would allow you to use something like this:

body .admin div#breadcrumbs.block {
  float: none;
}

Turns out the Firefox display bug affects blocks on non-admin pages as well so it might be better to just add this to your CSS and then look for anything it might break:

div#breadcrumbs.block {
  float: none;
}