What spirantho meant was HTML5 video support, via the WebM codec. MOS' OWB and Timberwolf support WebM, the OS4's port of MOS' OWB (MUI OWB) does not.
Just for the completeness:
Odyssey Web Browser supports HTML5 video and audio, and not only WebM (which is still not very widely used except on YouTube) but also H264, Ogg/Theora and FLV (and several audio codecs as well such as mpeg, aac and so on). FireFox given its choices will never support H264, which means it won't be able to view a good amount of HTML5 video websites such as DailyMotion or Vimeo and many others.
It currently scores 345 + 15 bonus points at html5test.com, by the way.
It also supports NPAPI plugins, but it wasn't adapted (yet?) on the OS4/AROS ports. Through this plugin system, Flash is supported with swfdec plugin (gnash would also be a possibility, but it's not better, anyway).
Finally, add-ons such as the ones you can find on userscripts.org are also supported by OWB (some needs some tweaking). These ones need the GreaseMonkey addon to work on Firefox.
