Ma Synaptic non ce l'hai?
E comunque al limite basta dare un semplice comando dalla console:


[questo è quello che vedo io su Sid]

# apt-cache search thunderbird | grep mozilla-thunderbird
mozilla-thunderbird - Transition package for thunderbird rename
mozilla-thunderbird-dev - Transition package for thunderbird-dev rename
mozilla-thunderbird-enigmail - Transition package for enigmail rename
mozilla-thunderbird-inspector - Transition package for thunderbird-inspector rename
mozilla-thunderbird-locale-ca - Mozilla Thunderbird Catalan language/region package
mozilla-thunderbird-locale-cs - Mozilla Thunderbird Czech Language/Region Package
mozilla-thunderbird-locale-de - Mozilla Thunderbird German Language/Region Package
mozilla-thunderbird-locale-el - Mozilla Thunderbird Greek Language Package
mozilla-thunderbird-locale-es - Mozilla Thunderbird Language/Region Package
mozilla-thunderbird-locale-fr - Mozilla Thunderbird French Language/Region Package
mozilla-thunderbird-locale-it - Mozilla Thunderbird Italian Language/Region Package
mozilla-thunderbird-locale-ko - Mozilla Thunderbird Korean Language/Region Package
mozilla-thunderbird-locale-nb - Mozilla Thunderbird Norwegian Bokål language/region package
mozilla-thunderbird-locale-nl - Mozilla Thunderbird Dutch Language/Region Package
mozilla-thunderbird-locale-pl - Mozilla Thunderbird Polish Language/Region Package
mozilla-thunderbird-locale-pt-br - Mozilla Thunderbird Brazilian Portuguese Language/Region Package
mozilla-thunderbird-locale-ru - Mozilla Thunderbird Russian Language/Region Package
mozilla-thunderbird-locale-sv - Swedish language package for Mozilla Thunderbird
mozilla-thunderbird-locale-tr - Mozilla Thunderbird Turkish Language/Region Package
mozilla-thunderbird-locale-uk - Mozilla Thunderbird Ukrainian Language/Region Package
mozilla-thunderbird-offline - mozilla thunderbird offline extension
mozilla-thunderbird-typeaheadfind - Transition package for thunderbird-typeaheadfind rename