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Iceweasel in Knoppix 7.2 is upgradable to Iceweasel 24.3.0 right now with Synaptic;
that's equivalent to Firefox 24.

Iceweasel 24 passes an html5 test with a score of 432 out of 555 points;...

Some background on html vs flash.
http://blog.accusoft.com/2012/october/html5-vs-flash-what-do-you-need-to-know-part-1
has its own link to part-2

You might like to give Firefox 28,
or another browser, a test:
http://html5test.com/

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Firefox 28 beta is available at
http://www.mozilla.org/en-US/firefox/channel/

This beta purports to transform Iceweasel to Firefox 28 beta capability.
That is, it replaces flash with html5. ...