> For instance in GMail I can't use Cmd+arrow keys to jump to the beginning or end of a line
This is a bug in gmail, not a bug in Firefox. In standard browser text widgets, Firefox reads your standard OS X key bindings and treats them properly.
However, Google is unsatisfied with standard form fields, and so they reimplemented their own glitchy, poor-performance, half-broken text widget, which happens to semi-work in Webkit, but not handle shortcuts like cmd-arrows in Firefox. It’s in no way fair to blame Mozilla for that.
This is a bug in gmail, not a bug in Firefox. In standard browser text widgets, Firefox reads your standard OS X key bindings and treats them properly.
However, Google is unsatisfied with standard form fields, and so they reimplemented their own glitchy, poor-performance, half-broken text widget, which happens to semi-work in Webkit, but not handle shortcuts like cmd-arrows in Firefox. It’s in no way fair to blame Mozilla for that.