There also seems to be an extension for KDE5, I haven't tried it but according to the reviews it should work:
https://addons.mozilla.org/en-US/firefox/addon/kde5-wallet-password-integrati/
Unfortunately you will have to fill your passwords in all over again, but you only have to do that once and then you can use the Firefox passwords across different profiles and different browsers - as long as they are based on Firefox and you install the extension, of course.