The First Ones seemed underdeveloped, and since Into The Fire would've been 2.5 episodes, is there any confirmation that the First Ones would have been explored more in these episodes had they not have been forced to create a logical ending point for the series (and therefore remove some details)?
I don't know whether the First Ones would have been explored more in Babylon 5 itself without the chaos that bedeviled the later seasons. However, I can say that J. Michael Straczynski indicated that there were plans to reveal some additional information about them in sequel series set in the Babylon 5 universe. At a talk on May 4, 1998 (between the end of production on Babylon 5 and the premier of Crusade in 1999), Straczynski talked about some of his plans for the future—including indicating that some topics that had gotten short shrift in the later seasons of Babylon 5 would be explored a little more if he had time. The First Ones were among the subjects that said he hoped would be given more detail.
However, he did not reveal any details about his plans for the First Ones, or indeed any specific topic—even when pressed by members of the audience during the question and answer session. His response (loosely paraphrased) was, if you wanted to know more about the First Ones: Keep watching. I personally doubt that, given their enigmatic nature and status as distant precursors, there would have been that much more information revealed. Explaining too much about the First Ones would probably have made them less interesting, and Straczynski (in that same talk, as well as elsewhere) consistently emphasized that he often didn't like to explain things too fully, because it felt like "talking down" to his audience.
JMS commented specifically on 'Into the Fire' - Q&A on the Lurkers Guide:
Are you happy with having to hurry season four along in case the show isn't renewed? Truth: I go back and forth. The "Into the Fire" thing, for instance... it would've likely been a two-part episode, but it still would've ended up exactly where it ended up. A few more big explosions, but I wonder also if that really adds anything past a certain point.
It suggests (but doesn't explicitly confirm) that the intent was to wrap the arc up in a similar way. There might have been odd details about the First Ones, but probably not significant revelations about the B5 universe