I had some difficulties in Ubuntu with installing a new spell check dictionary for my mail client Thunderbird. My native language is Dutch (Nederlands) and writing Dutch with a English spell checker active is not very interesting. I tried the methods explained at http://www.mozilla.com/thunderbird/dictionaries.html , but that didn't work. I got some message that the dictionary "has been succesfully installed", but that message did not reflect the truth.
I messed a bit with the installation script inside the spell-nl.xpi file, and found out that the dictionary would/should be installed to /usr/lib/mozilla-thunderbird/components/myspell, a directory that is only writable by the root user. Apparently the installation procedure did not detect the write failure. I could put the Dutch dictionary there as root user, but I looked a bit further and found out that installing the appropriate MySpell package (myspell-nl for me) is a much cleaner/safer solution. MySpell is the spell checker of OpenOffice.org, which is also used in Thunderbird.