Here is the ABNF description from RFC 6350:

  contentline = [group "."] name *(";" param) ":" value CRLF
    ; When parsing a content line, folded lines must first
    ; be unfolded according to the unfolding procedure
    ; described in Section 3.2.
    ; When generating a content line, lines longer than 75
    ; characters SHOULD be folded according to the folding
    ; procedure described in Section 3.2.

  group = 1*(ALPHA / DIGIT / "-")
  name  = "SOURCE" / "KIND" / "FN" / "N" / "NICKNAME"
        / "PHOTO" / "BDAY" / "ANNIVERSARY" / "GENDER" / "ADR" / "TEL"
        / "EMAIL" / "IMPP" / "LANG" / "TZ" / "GEO" / "TITLE" / "ROLE"
        / "LOGO" / "ORG" / "MEMBER" / "RELATED" / "CATEGORIES"
        / "NOTE" / "PRODID" / "REV" / "SOUND" / "UID" / "CLIENTPIDMAP"
        / "URL" / "KEY" / "FBURL" / "CALADRURI" / "CALURI" / "XML"
        / iana-token / x-name
    ; Parsing of the param and value is based on the "name" as
    ; defined in ABNF sections below.
    ; Group and name are case-insensitive.

(My emphasis.)

So, if I have the appropriate specification and I have read it properly, lowercase is allowed. I think you need to take your argument up with Google, because if they are refusing lowercase names, they are bucking this standard. I don't think Thunderbird is doing anything wrong.

This article shows up a general lack of standards compliance. Google specifically don't play nicely.

Thanks, yes guessed this might be the situation but wondering what the odds are on Google doing ant thing about it. Hey Google I can' export my address books from my Thunderbird to my android please change your code... Will give it a shot but expect I will just have to continue changing all the tags to uppercase by hand for the foreseeable future... :(

I was wondering how to automate it. You'd need to think carefully about what regular expressions to use to modify just the names parts.

Notepad++ has some useful tools and knows regular expressions…

Thanks Zenos, yes I can perhaps make something in bash for it, have not used regex for ages.

MoreFunctionsForAddressBook exports contacts and address books in vCard format with upper case tags.

To install the add-on, drag the Download link at the bottom of the page and drop it onto Tools/Add-ons/Extensions in TB.