I've been running the latest iceweasel from Debian Testing and have never had an issue with an upgrade. Then again, I'm running an extremely stripped down knoppix and there may be something in vanilla Knoppix that would interfere with keeping Iceweasel up-to-date. Regarding Apt configuration: The default Knoppix sources.list includes, among others, both Debian Stable and Debian Testing, along with Knoppix's own repository. You might want to clean it up some, but leaving it as the default shouldn't cause any problems. What might cause problems is a line in /etc/apt/apt.conf.d/00 knoppix(filename typed from memory, maybe slightly off) which sets the default realese for apt. By default, the line in question looks like: apt::default-release "squeeze"; In Knoppix 4.2.0. Squeeze is presentally Debian Oldstable, which isn't in the sources.list, and most APT frontends will throw an error and refuse to function if there isn't a matching sources.list line for the default release. if you change squeeze to jessie(for Debian Stable) or stretch(for Debian Testing), this problem will be fix. This line acts as a limiter to what updates will be installed. If you remove or comment out this line, your package manager will prefer the version with the highest version number regardless of it's source, while if this line is present, your package manager will prefer the version for the designated release unless the installed version is already of a higher release number. In other words, if the default-release is set to Debian Stable, updates from Debain Testing or Knoppix will only be installed if you force them or you're already running a version that looks newer than the version in Stable. Note: I may have gotten squeeze and wheezy mixed up(one is presently Debian oldstable, the other Debian oldoldstable), but in either case, you'll want to change it to Jeesy or stretch depending on whether you want Debian Stable or Debian Testing as your default release. I hope this helps.