You can get the latest Firefox build from the launchpad ppa repositories, but I found that the periodical beta release on Mozilla's website tends to be more stable and usable than the daily builds.
Unfortunately the downloadable build is only 32bit, thus not compatible with the installed plugins in Ubuntu 64bit version. After a bit of research I found out how to solve this little problem.
First you need to download the Firefox beta. Then
untar it to either you home folder or to
/opt/ (I like this way better, but you have to use
sudo to copy files here). You can
cd to that folder and launch Firefox (use: ./firefox or ./firefox -P to be able to create and/or choose your profile, more on that here)
As you can see there are no plugins in the add-ons page. Copying the plugin folder from the default installation does not work (duh!). So I thought to myself: this is some kind of standalone Firefox installation, then I would need a standalone version of Adobe Flash. And I found it here: http://www.adobe.com/support/flashplayer/downloads.html. There is a Linux version of the Flash Player 10.1 Plugin content debugger, as it is called. Download it and
untar it to the
plugins folder in your Firefox installation.
And there you have it, Adobe Flash Player in Firefox 4 beta. (Please keep in mind that both Firefox and Flash need to be updated manually if installed this way)