Martin (1992: 553):
The advantage of the dynamic perspective is that choices can be conditioned by the point reached in a text's development. Keeping in mind Firth's comment that "The moment a conversation is started, whatever is said is a determining condition for what, in any reasonable expectation, may follow" (1935/1957: 31-2), this is an important perspective to keep in mind.
Importantly, but apparently unknown to Martin, this reflects one crucial difference between Firthian Linguistics and the (Neo-Firthian) Systemic Functional Linguistics of Firth's student, Halliday. For Firth, consistent with his quote above, entry conditions to systems of choice are specified syntagmatically, whereas for Halliday, entry conditions to systems of choice are specified paradigmatically. Systemic Functional Linguistics, as the name implies, prioritises paradigmatic system over syntagmatic structure.