NoScript, which is one of the most popular add-ons for Firefox, allows you to only run Java and other executable content from domains that you trust. This greatly enhances the security of the Firefox's browsing experience. Usually, add-ons and extensions can be the cause of browser instability. However, this time, it's the OS that seems to be causing the problems.
According to Mozilla Links, the author or NoScript, Giorgio Maone has pointed out a problem with Ubuntu 10.04. The latest release for the popular Linux distro is causing Firefox sessions to crash when the NoScript add-on is installed. He notes that it may even happen without the add-on installed. NoScript just causes users to encounter the bug more frequently. The problem, along with a fix, according to his understanding is as follows:
"I'm receiving several reports by Ubuntu and Debian users having their desktops crashed as soon as they click the NoScript icon. Yes, your whole session gone, back to your logon page in one move!
Actually this may be triggered by different actions, not necessarily with NoScript installed, or reportedly just by having a long/complex enough interaction with Firefox.
It turns out to be a bug in the xorg-server package, which Ubuntu's maintainers deemed not worth to get fixed before releasing Lucid Lynx.
My understanding is that a fix will be included in next automatic update, but in the meanwhile you can apply the available patch from Bryce Harrington’s PPA by issuing the following commands in a terminal:
sudo add-apt-repository ppa:bryceharrington/purple && apt-get update && apt-get upgrade"