OK, sorry for being late in replying. The OS: Windows 8.1 64 bit, as security I only use Avast! free antivirus and as addons: Lastpass with xMarks, adblock and dnt (do not track). Did not try to unisntall/reinstall, as a main browser I use Chrome, but after being a FF addict in the past, decided to give it another go. Same addons on Chrome, but no lagging or feeling heavy.
As for Opera, well, used it a few weeks, maybe since they have decided to switch but...meh...really dont know what to say.
Have a nice day!