Ok heres my take on the next Firekkan campaign... take what ever you like from this and add it in. Though peaceful, the Firekkans are somewhat backwater. They are only starting to build their own craft so yes, what forces they have should be a mix of older, retired, confed/Landriech ships, perhaps civilian purchased vessels and their own craft. They are so despirate to defend their home, that they'd buy anything they could get thier hands on... except for Kilrathi ships as they believe those to have a stain of blood on them. The old confed ships should perhaps be repainted and rather decrepid looking covered with second rate replacement parts and very inferior armor. Give them superior numbers as you are in their home system. However, despite their overwhelming size, they have a massive disadvantage with the older ships only being able to take two, maybe three hits before going up in smoke. Perhaps give them an old Confed cruiser or something to use as a mobile base in addition to their own HQ.
As for an attitude... give the Firekkans their own code of honor. Not so much as a warrior race, but as defending their homeworld means more to them then their own lives. Perhaps some variation of Bushido. They are, as before mentioned, Xenophobic and they do not want anyone else in their system, ally or enemy. If their homeworld is to be saved, they have to be the ones to do it, or die. They don't like the idea of asking for help, even if they know they'll need it.
Thier ability to fight off the Nephilum on their own shouldn't exist. Like perhaps your first foray in to the Firekka system shows massive amounts of Firekkan craft getting blasted away by the Neps and I mean totally slaughtered with maybe one or two losses to the Neps. Your forces would first need to save what is left of their squadron. As you clear out the Neps, the Firekkans turn their weapons and give you moment s to explain your presence in their space before they blast you. At this point after the slaughter of the war, the Firekkans have become somewhat Xenophobic. They aren't trusting of anyone outside of their space, especially those who have Kilrathi on their wings.
After a short band of accusations going back and forth (including a shouting match of threats between the Firekkan and Kilrathi leaders), your CO agrees to leave their space, despite knowing that they can't properly defend themselves. As you are escorting your task force out of Firekkan space, you get a priority distress call from the single Firekkan capship (perhaps a Waterloo class cruiser). You race to their location and save their cruiser, only to recieve multiple distress calls from throughout the system, other squadrons, bases, and even their planet is under attack. Your squadrons spread out and attempt to save as much as you can. Your forces manage to save the main HQ the Firekkans built.
Their main HQ should look like something the wookies would built. Somewhat archaic looking. The squad saves the base, but thousands of Firekkans died in the attack on the planet. At that point, the Firekkans agree to let you help push the Neps out. Win or lose, you manage to push them out... if you lose, the Firekkan system is laid to waste. But if you win, the Firekkans will enter in to a new treaty with the Confederation. Even consider having a wing of Kilrathi ships rescue a squadron of Firekkans. Perhaps have a small discussion between your Kilrathi allies and the Firekkans After the Neps have been pushed out.
Firek leader: We are... in your debt, Kilrathi...
Kilrathi Leader: You fought honorably, worthy of notation in the Tomes of Sivar.
Firek Leader: You as well... I believe we have more to discuss then putting aside ancient hatreds...
Kilrathi Leader: We will be honored when that day comes.
As you escort your fleet out of the system, a wing of Firekkan ships (3 of the fighters you designed) confront your ships and the leader asks if his wing can join you. Now the Firekkan fighters are available for use and three pilots join your now dwindling squadron.
With that, your fleet leaves the sector.
Another thought, perhaps the end of the game should be one decisive fleet battle with combined Kilrathi and Confed ships, with the Firekkan Waterloo cruiser and fighters showing up at the last minute requesting to join your fleet in order to remove the Neph threat from Confed space once and for all.
I should really write a book on this campaign... yes? Maybe?