The spelling in the Cambridge manuscript is uniformly thire, except once when it is thir; and where their once occurs in the writing of an amanuensis the e is struck through.

That the difference is not merely a printer's device to accommodate his line may be seen by a comparison of lines 358 and 363 in the First Book, where the shorter word comes in the shorter line.

It is probable that the lighter form of the word was intended to be used when it was quite unemphatic.
Contrast, for example, in Book iii.

l.59: His own works and their works at once to view with line 113: Thir maker and thir making and thir Fate.
But the use is not consistent, and the form thir is not found at all till the 349th line of the First Book.

The distinction is kept up in the Paradise Regain'd and Samson Agonistes, but, if possible, with even less consistency.