Try clearing Firefox's cache. If the icon has gone missing from your WebIF installation, Firefox will then look the same as Chrome.All is fine if I use Firefox.
<span class="iconset">
<img src="/img/dedup.png" class="va" height="21" alt="Auto-dedup" title="Auto-dedup">
<img src="/img/tvdb-sm.png" class="va" height="21" alt="TheTVDB" title="TheTVDB">
<img src="/plugin/detectads/img/detectads.png" class="va" height="21" alt="Auto DetectAds" title="Auto DetectAds">
</span>
Confirmed: when I enabled adBlock plus the icons disappeared.Speculation: Could it be an over enthusiastic ad-blocker objecting to 'ads' in the image name?