When you have about:preferences#search opened in a tab, the icons will disappear when you navigate to another tab and then switch back to the #search tab after about 10-15 seconds. Hovering over the lists with the mouse will re-populate the icons. Issue: https://s3.amazonaws.com/otherDocuments/issue2.gif There's no clear STR, I usually use the search UI really extensively and wait about 1 minute or so before switching to the about:preferences#search tab. Eventually, you'll get to a state where it happens within 5-10 seconds. I could only reproduce this on my OSX 10.10 machine twice as it's really hard to reproduce.

Kamil, is this something that can still be reproduced in current builds?

I tried reproducing the issue with the folowing builds: - https://archive.mozilla.org/pub/firefox/nightly/2015-09-17-03-02-29-mozilla-central/ - https://archive.mozilla.org/pub/firefox/nightly/2015-09-17-00-40-25-mozilla-aurora/ - https://archive.mozilla.org/pub/firefox/candidates/41.0-candidates/build2/ - OSX 10.10.5 x64 -> couldn't reproduce - Win 10 x64 VM -> couldn't reproduce