Cache, Files, Cookies, Pretty Much all clutter, May slow down internet access, But usually Firefox can be a memory Hog, which would explain the delay when typeing. Try to free up some memory, and don't open to many tabs, 3 tabs can of over 60mb + of memory ontop of explorer 20k and the other running software like anti virus etc.. It can be problematic
You may want to look into going IE6 if you dont have many resources, it doesnt hog as much as others and the tabs like mentioned can overload you without even noticing.
Although your correct in the fact that IE6 does not use many system resources, I would not recommend it at all! I used to use IE6 and even with my very long background in computers, I always managed to get a fair amount of adware, spyware, and other annoying clutter on my system from the use of IE6... Firefox may be slow on low-end systems but it's certainly worth the added security that it brings to you compared to IE6...
To speed up Firefox, I'd recommend using very few tabs, deleting your cache each time you close Firefox. Also, if you know what your doing, there are a few options you can optimize in Firefox by going to about:config.