Now, after 5 months of development for Firefox extension of Simple Gmail Notes, most features are stabilized and the features could be shared between Chrome and Firefox easily. In the last few revisions, the cross-browser migration (Firefox -> Chrome or Chrome -> Firefox) all took less than 1 hour. Basically, the initial pain on the development of extension development gradually passed away.
However, I would like to further discuss the review process of Firefox. As mentioned before, Firefox uses a manual review process, it is much more strict than Google Chrome extensions (which is, basically, nil).