Try this, edit nsLoginManager.js, might live in a different place depending on your OS, but for me it lives in /usr/lib/firefox-#.#.##/components/nsLoginManager.js and find the function _isAutoCompleteDisabled, and change any “return true” statements to “return false”. Or if you’re feeling adventurous (i.e. you aren’t worried about having to revert) you can just make the whole function “return false”. Fixes this behavior on a few websites for me. Of course these sites usually do not remember passwords as a security precaution, so you have been warned.
For me, I do this, and then find the line of interest
find /usr/lib -xdev -iname nsloginmanager.js 2>/dev/null|\grep firefox|xargs sudo gedit