1. Inspect <body> on this Bugzilla page.
2. Add new rule by clicking the plus button at the bottom of the styles sidebar.
3. Click on the "Inspector Style Sheet:0" link.
"Inspector Style Sheet" resource opens with the newly added CSS rule in it.
Resources tab opens with some other resource selected.
"Inspector Style Sheet" isn't present in the left sidebar.
*** This bug has been marked as a duplicate of bug 188819 ***