firefox.desktop and thunderbird.desktop are not written in the same style.
Comment=Browse the Web
Name=Mozilla Thunderbird Mail/News
Comment=Read/Write Mail/News with Mozilla Thunderbird
I like the very short Firefox strings and dislike the very long Thunderbird strings.
I suggest changing thunderbird.desktop to something like:
Comment=Read and write Mail/News
Maybe fix the order of lines starting with the following so they come in the same order in both files: