Few of my friends are facing problem with firefox 4.To be truthful its very few but its there.One of such problem is the warning-"your browser does not support WebGL".There is definitely a way to solve this.
Just open the synaptic(System>Administration>Synaptic) and search for "libosmesa6".You'll get 2 results(just look at the screenshot),select the file(the 'dbg' file is not needed for regular use) and 'apply' it.
Now open firefox 4 and type "about:config"(without "") in the url bar.search for "webgl.osmesalib" and add a string type "/usr/lib/libOSMesa.so.6".
Restart the browser.
To check whether webgl is actually working or not,try this.