Sleazy Fork is available in English.
Can work on almost any webcomic/manga page, preloads 5 or more pages ahead (or behind), navigates via ajax for instant-page-change, lets you use the keyboard, remembers your progress, and it's relatively easy to add new sites
nhentai.net not working
The script seems to have stopped working on nhentai again, tried fiddling around with it myself but not quite sure what changed.