MIDEAST STOCKS-Gulf mostly firm; Egypt regains some ground

* Abu Dhabi's NBAD retreats, snapping two days of big gains * But UNB, ADCB jump on speculation about another merger * Saudi loses steam as Brent oil slips back below $50 * Kuwait's Americana extends rise towards acquisition price * Black market currency moves worry Egypt By Celine Aswad DUBAI, June 21 (Reuters) - Most Gulf bourses firmed on Tuesday as fresh merger speculation moved stocks in Abu Dhabi, though Saudi Arabia's stock market lost steam as oil prices fell back. Egypt's bourse partial