I tried to get the armel build of Firefox, but nothing. So Ubuntu actually doesn't have armel binaries in its repo. You have to add the Debian repo (and remove it after!)
Furthermore you have to add --reinstall option if you download all the dependencies, even if you have already installed some of them to your current machine (in this case you should not have any problem, since I don't think you're using an ARM PC, but you never know
)