Comment 9 for bug 158880

Had the same issue.

F-spot sends a string like this to the default mail program:
mailto:?subject=my%20photos&attach=/tmp/somefile.jpg

_For attachments_ Thunderbird wants to be called like this:
 thunderbird -compose "subject=my%20photos,attachment='file:///tmp/somefile.jpg'"

I dont know who should behave better, but clearly this is different.
Also, if you set Thunderbird as your default, it will be called without
the -compose option, and it will fail _for attachments only_

Good News:
I have a workaround in /usr/local/bin/thunderbird-mailto.py
(sorry dont remember where from)
Then, in Sytem->Preferences->PreferredApps I entered
the following custom command as default mail
 "/usr/local/bin/thunderbird-mailto.py %s"

Works for me.
Good luck.