Comment 8 for bug 1016996

No need to use xmodmap in Unity or mess around with the Xorg.conf file. Delete your .Xmodmap file.

Open System Settings (be warned it may take several minutes or even hours before the popup finally appears even on a fast system)
Select Keyboard Layout
Select Options
Select Compose key position
Select Right Alt
Close window

The AltGr (right Alt) key works fine in firefox.
Type simultaneously AltGr + ','(comma) and then quickly type 'c' to get ç (if you type too slowly, you will get a plain 'c')
AltGr + ':' then 'e' for ë
AltGr + ' then 'e' for é
etc