I apologize for the incredibly late response to this.
No, as far as I am aware it is not possible to attach to an instance of Firefox that Selenium-WebDriver itself didn't start up.
One reason to _want_ to do this is to use your personal Firefox, for example, with all its settings and plugins installed. Selenium-WebDriver Firefox support allows enabling plugins that you have installed as well as a number of other things. So you might look in that direction if you need more than the bare-bones Firefox profile that Selenium-WebDriver boots up by default.
Otherwise, if you're experiencing problems with the time it takes to open and close your browser, then I find it helps to write your tests such that your browser(s) stay open for as long as possible, and you simply write test fixtures to bring the browser back to some clean/default state for each test in your suite.
-Daniel