Having this problem, too, but I have a feeling this has something to do with how Capital One has coded their site, not something LastPass has done. I've encountered no less than 4 different login forms all set up completely differently on Capital One's website, and while I did get it working several months ago by tracking the "name" attributes of each username/password field and updating my entry in LastPass accordingly, it looks like some sort of script is running in the background of Capital One's website which is interfering with the autofill capability of LastPass -- only in Firefox, it's worth noting, so that's an issue for Mozilla, too.
Having gone through the scripts in use by capitalone.com using uBlock Origin, I blocked all scripts on deviceinfo.capitalone.com and LastPass began working again on Capital One's website. (It seems after further review only one script -- cc.js -- is hosted at this domain. I do not know what it does, the code has been minified.) This script IS also loaded in Chrome, so why it's breaking in Firefox is, again, an issue for Mozilla. But if you have a robust adblocker like uBlock Origin, target deviceinfo.capitalone.com for blocking and you'll be fine... until Capital One changes their disaster of a website backend again.