Micro-optimization for browsing experience, part 1.

[+] Changed preference 'mousewheel.min_line_scroll_amount' from 5 to 40.
[+] Changed preference 'general.smoothScroll.pages' boolean value from true to false.
[+] Changed preference 'image.mem.min_discard_timeout_ms' from 10000 to 1050000000.
[+] Changed preference 'image.mem.max_decoded_image_kb' from 51200 to 512000.
[!] Preference 'general.smoothScroll' should be set from true to false.
[!] This commit aims to improve browsing experience, on sites such image galleries like DeviantArt, Pixiv, etc.