"Firstname" or "First Name"?

It should be two words: "First Name"

There does appear to be an upward trend of lumping the words together, but it's still pretty small - and incorrect. I suspect some factors in that trend include:

• Popularity of "username" leading people to think that all such uses can be smushed together into one word.
• Accidental or automated use of naming conventions from variable names or database tables (which will usually not have spaces in them).

Is it generally acceptable to join the words like that?

I think we are seeing the language evolving. The technology behind the webpage almost always uses firstname and lastname in the respective variable names. Many programmers (including those who are raised and schooled in the US) do not have strong English skills, and they are heavily exposed to the single word variation.

I'd argue it isn't wrong, it's just a new variation that may someday become acceptable.

I think the reason why it is written a lot like "firstname" is because in other languages first name is often one word (ex. dutch, french, german, Danish, Fins, Greek, ...). A lot of websites and forms are translated from other languages into English to go international. Everyone can read English. :)