You can build and update i2pbutton without the need of recompiling firefox for most tasks luckly :)
To do this build it and copy it into your data directory:
./makexpi.sh
cp pkg/i2pbutton-0.2.xpi ~/I2PBrowser-Data/Browser/914o5i1s.default/extensions/i2pbutton@geti2p.net.xpi