The recommended way to browse the web through Tor is to use the Tor Browser Bundle. Using any other browser will make you different from the common anonymity set of Tor users.
Changing the configuration of the Tor Browser is not recommended exactly for the same reason: it is likely to make the browser behave differently from everyone else's.
Tails include a browser which supports both Tor and I2P using FoxyProxy. See the design document for more information. As Tails' user form their own anonymity set, it's a way to use both protocols in a single browser.