firefox.desktop and thunderbird.desktop are not written in the same style.
Compare https://dev.getsol.us/source/firefox/browse/master/files/firefox.desktop$3 and https://dev.getsol.us/source/thunderbird/browse/master/files/thunderbird.desktop$3
Name=Firefox
GenericName=Web Browser
Comment=Browse the Web
Name=Mozilla Thunderbird Mail/News
Comment=Read/Write Mail/News with Mozilla Thunderbird
GenericName=Mail Client
I like the very short Firefox strings and dislike the very long Thunderbird strings.
I suggest changing thunderbird.desktop to something like:
Name=Thunderbird
Comment=Read and write Mail/News
GenericName=Mail Client
Maybe:
GenericName=Mail/News Client
Maybe fix the order of lines starting with the following so they come in the same order in both files:
Name
Comment
GenericName