Bizzarely, Microsoft Word on web seems to be the only thing I've encountered which has FireFox problems, specifically it'll periodically "save" the document I'm typing and then delete the last sentence or two that I changed while it was saving. I think it's some kind of broken state management on the MS side (leave it to MS). That's the only site I've used though, and I've been a daily driver of FF for 10 years.