Sorry, but I don't understand with "firefox browser has got bigger". The icon, the browser window, the characters inside the browser window?
Anyway, it's quite possible also that you changed your screen resolution. From, say, 1024x768 to 800x600. That makes EVERYTHING look bigger. And it's a totally different setting: http://www.helpwithpcs.com/tipsandtricks/changing-screen-resolution-win-xp.htm
Hope this helps.