Hey there!
NetRunner 15 installed. If I have Firefox and Thunderbird windows opened and press Windows key->Power / Session->New Session, when I relogin and try to launch these apps, I get “Firefox is already running, but is not responding” message. Same for TB.
Is it KDE bug?
Thank you!
NO it’s not a KDE bug, it a Firefox, Thunderbird issue. These applications have multi-user support built in via it’s own mechanism. They are not designed to run more than one instance at a time by default. Even though your opening them in a different session, the application is already loaded into ram and will not load another instance.
So, what’s the proper solution? Force KDE to kill them on new session or make them somehow restore to new session or run new instances (which seems to me a potential for misbehavior)?
There might be a setting in Firefox itself to allow more than one instances at a time, I don’t remember as I haven’t used Firefox in a very long time. 
What do you use, anyway? 
Google Chrome from the AUR (Arch User Repository), However under Netrunner 15 (Ubuntu) you would need to add the Google repository from their site by selecting download Chrome, and choosing either 32 or 64 bit Debian/Ubuntu:
https://www.google.com/intl/en/chrome/browser/desktop/index.html#brand=CHMB&utm_campaign=en&utm_source=en-ha-na-us-sk&utm_medium=ha
Oh, btw, sorry for going off-topic, but what kind of graphic card do you have? I’m asking because I have nVidia, installed proprietary drivers, still in both VLC and Flash (both Firefox & Chromium) I have that feeling that video is not smooth. In VLC its like refresh thing, in Flash - totally slow on full-screen. Tried many hacks, except for APT::Cache, but not use - still not as smooth as it was with Windows.
Intel on Intel, Sorry, but I’m one of those guys that doesn’t like closed source blobs on my system.
If your not into gaming. have you tried using the nouveau drivers with that card?
If you are then never mind, LOL.
Also did you install any video playback acceleration packages like:
vaapi, libva, libva-mesa, libvdpau, libva-vdpau, mesa-vdpau, libva-vdpau-driver, libvdpau-va-gl, etc.
I’m not sure what the Ubuntu packages would be since I’m an Arch, Gentoo, Netrunner Rolling (Manjaro), KaOS, etc. type of guy.
You could also try to avoid flash whenever possible. It’s not updated anymore besides security fixes and never was a performer.
As for browser I use chromium the opensource variant of chrome. It comes with some less NSA ähm… Google spying services by default.
If you want flashplayer there you need the ppapi flashplugin that you can find via synaptic/muon.
As for firefox itself there is no real solution besides closing it everytime you want to switch a session.
@AJSlye, libvdpau worked 95%. Thanks!! The rest 5% went from Compositor option “vsync” set to “Full screen repaints”. Note: I’ve tried experimental feature “turn off compositor for full screen applications”, but it actually creates greenish flickering when moving mouse (eg. showing VLC panel).
I don’t mind a little closed source. I believe if we fail to detect the enemy, he’s just too good, and regardless of what technology we’re using, he’ll find a way.
+1 for Arch. Gentoo seems to me overkill (compiling on your own? unless you manually check each line of code for possible vulnerability or back door, I’d say it’s a waste of time).
@leszek +1 for avoiding Flash. It is the future, but still not 99% adopted.