There is a bug in the current flash player for Linux which, in Ubuntu, will cause FireFox to crash during YouTube videos. Now, a quick solution to this is to simply remove the package libflashsupport, but this keeps you from being able to use Flash with Pulseaudio, which lets you have multiple things using the sound card at the same time (some sound cards can do hardware mixing, but a lot can’t, hence the need to do it in software with the sound server). Overall this is very annoying, especially for a desktop OS.
There is, however, a solution. Basically, you need to update a bunch of packages to the versions intended for the next release, which is Intrepid Ibex (at the time of this writing). There are details on the problem in this thread, and you can go to this thread for the fix. Here is the gist of it, though. Just remember, while this worked for me, do this at you’re own risk:
First, stick the following two lines into your /etc/apt/sources.list file:
deb http://ppa.launchpad.net/psyke83/ubuntu intrepid main
deb-src http://ppa.launchpad.net/psyke83/ubuntu intrepid main
Next, enter the following at the command line:
sudo apt-get update
sudo apt-get upgrade
Just accept both packages; it will have to uninstall your current versions too. The above worked for me; be aware of the package names in case you need to uninstall them. They should be listed on the thread I linked too.
Another interesting thing to do is to run the flash plugin within nspluginwrapper, which is usually used to let you run it on different architectures like PPC or x86_64. The first thread I linked to has a link for a specially-compiled 32 bit version of nspluginwrapper, allowing you to run it on normal x86. This makes Flash run as a separate process, so that if it crashes, it won’t bring down FireFox. Also, as it runs as a separate process, it runs faster on multicore/multi processor machines.
Hope this helps!