Rated 4 out of 5 stars
I only did some short tests with SeaMonkey German 2.39 final Mozilla/5.0 (Windows NT 6.1; WOW64; rv:42.0 from official download area) Gecko/20100101 Firefox/42.0 Build 20151103191810 (Classic Theme) on German WIN7 64bit for "Bug 1242294 - Advertise exactly Firefox when compatMode.firefox is true" <https://bugzilla.mozilla.org/show_bug.cgi?id=1242294>:
Core function works fine.
List of available user agents is difficult to find.
May be shipped user agents need an update? I can't imagine for what IE6 might be useful.