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The web server is probably only bound to the localhost interface as a security measure.
You could launch a remote firefox as mroth suggested, but I would use ssh port forwarding instead:
ssh "your_server" -L8080:localhost:80
Then you can open a browser with the url: http://localhost:8080/horde/
and that should do what you want.
Or you can open a SOCKS proxy server using ssh -D ip_addr and configure your local copy of Firefox to use that.
Firefox -> edit -> Preferences -> Advanced -> Network -> Connection -> select Manual proxy configuration: SOCKS v5