Theme developer (and AMO Editor) KLB reported this bug after some discussions we had at MozCamp EU. It was a regression caused by the fix for this other bug, which is just a minor toolbar update for Mac OS X Lion. The culprit bug fix includes the following override:
override chrome://browser/skin/Toolbar.png chrome://browser/skin/lion/Toolbar.png os=Darwin osversion>=10
This breaks themes that try to reference Toolbar.png, since Firefox will look in the different location (in the lion subdirectory). Possible workarounds include using a different file name for the toolbar sprite and change all references in the theme, or adding a duplicate or similar file in the location Firefox expects to find it.
We haven’t decided on any solutions for this problem, but you can follow this bug if you’re interested in the outcome.
And since we’re talking themes, theme developers should read Wladimir’s post about making themes more like overlays and less like overrides. It’s a very interesting topic that was brought up at MozCamp and that I’d like to discuss it more in the future. Having feedback from theme developers is very important to give this idea some traction.