I have been using Opera for many years since 6.x and except occasional page rendering issues (normally due to IE-centric coding on many sites), it has been my favourite because it is stable and fast. However, there is no doubt that Mozilla/SeaMonkey/Firefox is strong with a wide range of plugins available (that you previously mentioned).

Opera is still my favourite on Linux, although occasionally keyboard will be locked up probably due to some windowing toolkit issue (i.e. Qt).

Google Chrome also loads fast on my system. Often lighter compared with Opera so I tend to use it more often on Windows these days.