query to get concatinated data when using 'group By' without using group_cancat function

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ASP.NET

MySQL

hi all...
i have a chat table like this

id    msg      firstname    lastname
1      hi       rams         var
2      hello    rams         var
3      hi..     var          rams
4      h r u?   rams         var
5      m fin    var          rams
6      h abt u? var          rams
7      gud      rams         vat

here i need to get result like

rams:hi hello
var: hi
rams:h r u?
var m fin h abt u?
rams:gud

for this i have written query like:

select concat(firstname,':',msg,'<br />) as msg from chattbl group by firstname order by id asc

it results:

rams:hi 
var: hi

but here i need to concatinate all the data according to firstname of above table if i use group_cancat function here it limits the results set up to some limit but here user input may not have limitation .
can any one please help me....
thanks in advance....