Hi,
You get the output as @{concat('t1','</br>','@{activity('Lookup1').output.firstrow.FI_NM})}, because, you have enclosed the expression in double quotes.
Try removing the doubt quotes, curly braces and see if that helps. Like below -
@concat('t1','</br>',activity('Lookup1').output.firstrow.FI_NM)
Let us know how it goes,
Thanks