Comment 4 for bug 1329046

None of the major browsers will support a webapp mode though.

Firefox does not. Our initial approach at using an extension to implement that proved to be problematic. Even more so now that Firefox plans to require signed extensions by default.
Chrome was supporting a webapp mode. We could resurrect that, but that would create a situation where Firefox users don't have webapps, whereas Chromium ones do.
In the end, and in the perspective of convergence, using our own web engine is the most consistent. At least for modern sites which do not require Flash.

For older sites using Flash, users can still use their favorite browser, and put a bookmark on the launch bar.