This is how to do it:
Start up your browser from commandline and use the following arguments:
firefox -no-remote -P
-no-remote tells Firefox that the instance you are about to start up should not be connected with any other running instances.
-P tells Firefox that you want to use profiles. If none is given you will be asked to create one or use an existing. That's it.
This is quite smart as a development version often will use plugins (e.g. firebug) which can be a bit heavy on the user experience. Now you can separate this entirely.
Note: This is done on Ubuntu 11.04 with version 6 of Firefox.
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