Firefox crashed for some reasons. One example is when you try to open youtube video in full screen. How do you know that the Firefox crashed? Firefox just freezes and later kde pop up a message telling you that Firefox is not responding. Kde gives choices to terminate or keep running Firefox. When you terminated Firefox and open a new one, this error appears:

What you need to do is open command line terminal and switch user to root. You can use su - command and enter root password. If you are using Ubuntu, use sudo command instead. See examples below on how to get root privilege for Slackware and Ubuntu and run ps command to grab firefox process:

On Slackware:

luzar@hitam:~$ su -
Password:

root@hitam:~# ps aux | grep firefox
luzar    12167  3.9  0.0      0     0 ?        Zl   15:18   5:26 [firefox-bin] 
luzar    12561  0.0  7.7 405736 160172 ?       SL   17:30   0:00 /usr/lib/firefox-3.0.5/firefox-bin
root     12975  0.0  0.0   2088   656 pts/1    S+   17:35   0:00 grep firefox
root@hitam:~#

On Ubuntu:

luzar@hitam:~# sudo ps aux | grep firefox

We can see the result of Firefox processes from the ps aux | grep firefox command above, that the Firefox process id 12167 becomes a zombie and 12561 is asleep. Terminate the process and use -9 option to force terminate if necessary.

root@hitam:~# kill -9 12561
root@hitam:~# ps aux | grep firefox
luzar    13009  0.3  2.6  79120 55720 ?        Sl   17:40   0:03 gimp-2.4 /home/luzar/screenshots/firefox_crash.png

Now we can start Firefox again in kde. When we open Firefox again, we'll get this message from kde:

You can choose Start a new session or Restore previous session now, it's up to you.

One way to fix this problem is by unchecked Enable hardware acceleration in the flash player option. What you need to do is open youtube, right-click on the flash player and choose Setting. See the picture below:

This fix was suggested by Jan-c (
jannekrille) from LinuxQuestions forum and it works.