There are at least two addons available that support both Zawgyi and Unicode. So there was no need to develop an addon which duplicated those. Many Burmese websites use Zawgyi, yes that is true. But Zawgyi is incompatible with Ethnic minority languages used in the Myanmar script. This app was designed to support all Myanmar script languages in Unicode.
Burmese , Mon, Shan and Karen languages use a number of incompatible fonts based loosely on Zawgyi. They are incompatible. If I added support for Karen languages using their version of Zawygi, it would be incompatible in some cases with zawgyi and potentially corrupt Zawgyi based sites.