) because of all the different add-ons I have, each with its own script.
Of course, Firefox uses the Gecko engine, which, whilst very advanced and expandible, lacks the speed of Webkit based browsers such as Google Chrome (not yet on Linux), Safari, Epiphany etc.
Another possibility is process management, when I used Ubuntu I hardly ever felt the scripts weighing down my Firefox, on Pardus I use a lot more applications simulataeneously which slows down the browser ultimately. The other distros you have used most likely had better process management.
Lastly, the add-ons you install are very important, some should not be used on Linux at all due to compatibility errors, are there any add-ons you use now, that you didn't use when using other distros?
The next version of Firefox will have far better scripting control in an attempt to undermine Chrome.