Hey fellow forum person. I'm as displeased as the next user with not getting any updates on this yet from LP, but I stumbled across a weird trick to get around this.
Hopefully this helps the engineering team isolate and identify the issue, and other users, as a stopgap.
Apparently, if you wait for the page to fully load, auto-fill doesn't work. But, if you auto-fill before the page finishes loading, auto-fill works.
On Firefox 57, Linux, on the primary landing page, this trick worked for me for the following URL page and the following URL only:https://www.capitalone.com/
Auto-fill still does not work (not even when trying the above trick) on their newer login page:https://verified.capitalone.com/sic-ui/#/signin