The meadata plugboard I use on my original Kindle Fire is:
Code:
mobi:KINDLE_FIRE = ([{series:|| }{series_index:0>2s|| - }{title}] -> title)
This works for me because I have Preferences -> Output Options -> MOBI Output -> MOBI file type set to "old" and because I
always download the Kindle 1 version to my computer (so it is always an old mobi to start with, even if I don't convert it in Calibre).
In your case, most of the commenters are suggesting you replace mobi with azw3 in the above and then convert all your ebooks to azw3. This is in part because AZW3 is a more capable format than MOBI.
If you want the same file on your Kindle 1 and Fire then use the "old MOBI" approach, but if you have a mix of Kindle 1 and Fire sourced MOBI's in your Calibre library you probably still need to do a mobi to mobi conversion of all your ebooks.