I've had the same problem when running Avast Passwords plugin for Firefox (latest version) on Windows 10. I've tried pinpointing the problem and it seems that the plugin starts hogging massive amounts of RAM when you have Facebook open in Firefox. The same problem might happen with other pages as well but it's most noticeable with FB. My Firefox memory usage jumps from 300mb to over 2gb in less than a minute after opening FB and scrolling the tab for a bit to load up more items to FB newsfeed. Eventually the browser becomes unusable or even crashes.
EDIT: Modified because I initially wrote Avast EasyPass instead of Avast Passwords.