One bound to be partial to browser (Firefox) (4)
I believe the answer is:
serf
'one bound' is the definition.
(someone bound to serve in feudal system)
'partial to browser firefox' is the wordplay.
'partial to' indicates the answer is hidden within the clue (the answer forms part of the clue).
'SERF' is hidden in the letters of 'browser firefox'.
'to be' is the link.
(Other definitions for serf that I've seen before include "Bondman" , "One bound to the land" , "Subservient labourer" , "Medieval villein" , "Slave" .)