Feisty did use another /usr/lib/firefox/firefox script than gutsy does, no idea, why it changed. Gutsy version does not read /etc/firefox/firefoxrc and ~/.mozilla/firefox/rc anymore, the "hack" for enabling pango only for a certain set of languages is no longer applied, too. At the moment I'm using the /usr/lib/firefox/firefox script from feisty-package on my gutsy install to workaround this, I've had no time to investigate if this is causing some serious troubles, at least it seems to work fine for me.
Feisty did use another /usr/lib/ firefox/ firefox script than gutsy does, no idea, why it changed. Gutsy version does not read /etc/firefox/ firefoxrc and ~/.mozilla/ firefox/ rc anymore, the "hack" for enabling pango only for a certain set of languages is no longer applied, too. At the moment I'm using the /usr/lib/ firefox/ firefox script from feisty-package on my gutsy install to workaround this, I've had no time to investigate if this is causing some serious troubles, at least it seems to work fine for me.