It would be useful for users to be able to modify the Accept, Accept-Language, Accept-Charset, Accept-Encoding and Accept-Ranges HTTP headers. For instance Alexey has had trouble with some servers choking on languages with q-values, and wanted them removed, whilst I would like to specify my own values for (British) English, French and German, and do not come across servers that this causes a problem on. I also don't want to use 1.0, 0.66 and 0.33 which is what Firefox would assign the three languages if I put them in that order, but perhaps 1.0, 0.4, 0.1 !
Note also that most browsers (Opera being the primary exception) send a different set of MIME types in the Accept header depending on how the resource request was generated (e.g. the main resource might want to accept */*, whilst you'd be more choosy from an image, script or link element). See bug 3510 for more on this.
Since there is no "one size fits all" solution to this, I suggest that the headers are exposed via an API allowing UAs such as Safari to provide an interface by which the user can set their own values for these headers. Obviously this would be an advanced feature and suitable defaults still need to be decided upon, but it would create the opportunity for users to configure their systems for their own environment.
Sorry, that reference for more info on what browsers send what headers should have referred to bug 12296.