Chrome has been through 13 major releases in its 3 years, so FF is just moving to a schedule/numbering convention that Chrome has been following. 5 and 6 would have been 4.1 and 4.2 in the past - they are relatively minor.

FF4 was the last big release and most of my add-ons worked fine and were "officially" support either at release or within a few days.

There is a FF 7 "out", but it's a beta version. Heck, there's FF8 "out" in the nightly channel and FF9 features (tablet UI) have been announced. There is also a version 14 and 15 of Chrome "out" if that counts.

"current stable version" - I'd go to Help/About/Check for Updates and let FF tell you.