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
Navigation Suggestion
When the image has horizontal scrollbar, some sites set the next and previous key to double click of left/right. So left click or right click behaves like normal scroll. If the image is fits in normally then the left/right (next/previous) work on single click.
Would you consider it.