I sort of achieved this by 1) creating a separate profile for a web app, 2) create a Desktop File which points to that profile, with a `--class` option set to t

I sort of achieved this by 1) creating a separate profile for a web app, 2) create a Desktop File which points to that profile, with a `--class` option set to the app's name. It seems that the logins are staying for as long as the website allows. The caveat is that the `--class` option doesn't seem to always be reliable; sometimes the window still gets grouped with the normal firefox instances in the task bar. I haven't got around to find out whether it's the fault of firefox or kwin.