5b Ἰδοὺ ἡλίκον πῦρ ἡλίκην ὕλην ἀνάπτει· Behold how small a fire sets ablaze how great a forest!
6 καὶ ἡ γλῶσσα πῦρ And the tongue is a fire!
The unmarked word order of a verbless clause is for the subject to precede the predicate, as is the case here. Natural salience is assigned to the predicate, since the subject is typically established information and, also, what the predicate is about. So, normally, the predicate contains the most important piece of information. However, in this case, both tongue and fire have already been mentioned; they are, thus, established pieces of information in the discourse. So the question remains: what is this verbless clause really about, the tongue or the fire?
There are several observations we can note that lead us to believe that ἡ γλῶσσα (the tongue) is the topic of this verse and that πῦρ (fire) is the focal information. First, it is important to recognize that even though ἡ γλῶσσα and πῦρ were both recently topics in v. 5, the degree to which they have been established as topics is not the same. The former is by far the more established topical entity, with πῦρ being only recently introduced. Indeed, ever since v. 1 ἡ γλῶσσα has operated as an implied topic at a higher level, until it was explicitly mentioned in v. 5. The same cannot be said for πῦρ, which was only commented on for the sake of illustration. In short, it has not been a central theme like ἡ γλῶσσα.
Second, the only reason ἡ γλῶσσα could be considered the most important (i.e. unexpected) piece of information in this verbless clause is if an open proposition exists in which "X is a fire" has been evoked in the readers’ mind. If this was the case, then ἡ γλῶσσα should indeed be understood as filling this unknown and anticipated slot; and, as a result, reflect marked focal information. But frankly, it’s highly unlikely this open proposition exists. It is possible that the juxtaposed propositions in v. 5 could imply this, but the higher level status of ἡ γλῶσσα ("the tongue") as topic indicates that the reference to a forest fire in v. 5 only serves as a foil to introduce the comment under consideration. In other words, the point behind the forest-fire remark is not to begin a conversation about fire in which an open proposition of "What else is a fire?" might be evoked; it is simply to set the stage for a comment that the tongue is a fire in its own way.
The third reason, which supports the former, is that if James had wanted indicate that ἡ γλῶσσα was the focal information, there are at least two clear ways he could have demonstrated this. First, he could have dropped the article from the noun phrase ἡ γλῶσσα, which is a typical feature of focal constituents. Second, he could have inverted the word order, so as to place ἡ γλῶσσα in a marked position, after the predicate. However, James does neither; and instead leaves the constituents in their default order of topic-comment.
The final reason extends beyond the current proposition to the preceding analogy between the rudders of a ship in the tongue. In v. 4 and 5a James drew a comparison between the tongue and the guiding power of rudders on large ships. He pointed this connection out by explicitly marking the comparative frame with οὕτως(in this manner) as well as drawing more attention to it with an adverbial καί (also). The same puzzle pieces are present in v. 5b and 6, only here the explicit comparative frame marker is implied. This analogous setup provides weight to the fact that ἡ γλῶσσα is topical in verse 6, just as it is in verse 5.
And so, because of all these factors, it seems that the most natural reading is to take ἡ γλῶσσα as the entity that the proposition is about, with πῦρ as the focal information. After all, in v. 5 the tongue and fire were mentioned as elements of two distinct propositions. Even though an implicit relation is present, they were not explicitly connected in the same proposition—until now. And so it is this novel connection between ἡ γλῶσσα understood as πῦρ that creates an assertion, which entails that καὶ ἡ γλῶσσα πῦρ is appropriately understood as a topic-comment articulation.
Behold how small a fire sets ablaze how great a forest! And the tongue IS A FIRE!