Mozilla/5.0 (Windows NT 5.1; rv:33.0) Gecko/20100101 Firefox/33.0 Daily updated nightly build, old profile, no add-on installed. Steps to reproduce: 1. open in-content preferences (about:preferences) 2. select the 3rd sub-tab "Applications" 3. select any content type, e.g. "Portable Document Format (PDF)" the line is now highlighted and a drop-down list appears to choose the action 4. do not change any setting and close the browser tab 5. open again the in-content preferences 6. go back to the 3rd sub-tab "Applications" 7. select the same content type as in step #3 Actual results: The content type is not selectable, mouse clicks do nothing. No highlight, no drop-down list. Expected results: The content type should be selected: highlighted, drop-down list available. I think it has something to do with the current behavior of the preferences popup witch saves the last selected "content type" and highlights it back when reopened. There is no issue using the same build with a new profile. This problem might be more about profile migration than a behavior bug.

Comparing my old profile with a freshly created one, I find that "localstore.rdf" was quite huge: old=26.3KB vs new=2.22KB After deleting "localstore.rdf" from my old profile, Firefox recreated a new one and the issue was no more. So it is either a profile migration bug (hard to tell with a nightly profile) or INVALID/WORKSFORM.