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
Sorry for the late answer. You can try downgrading to an older version of both firefox and greasemonkey and exporting the data from that. That worked for this person at least: https://github.com/anka-213/webcomic_reader/issues/93
My bookmarks got deleted, need help.
Just yesterday my bookmarks got deleted after Firefox upgrade to v57 (quantum), I'd appreciate if someone can tell me the file in which the bookmarks are stored. I remember that it was in the profile folder, in a json file? Honestly I can't remember which exactly, but I can't find it again.