Whenever Selenium fires up Firefox (we are still running Selenium in RC mode) a new Firefox profile directory will be create at every startup. Usually this directory will be created in /tmp - which is for various reasons not always the desired location.
Selenium RC itself has no configuration option to change this location as
it relies on the default value provided by java. Fortunately java provides
a command line option
-Djava.io.tmpdir allowing to specify a new tmp directory.
So change your startup call of Selenium RC to
java -Djava.io.tmpdir=/your/tmp/dir -jar selenium-server.jar
and you're all set.