TBH, if the FB isn't expected to fight SCs, the Trenchgun and Mac10 combo is simply the best. Mac-10 for little stuff like clots, crawlers, and stalkers and Trenchgun for gorefasts, husks, and sirens. Mac-10 is also great for semi-auto for tight-controlled headshots, and the flare revolver in this case is simply not needed.
Flare Revolver(s) are significantly better than the MAC-10, and it isn't even close. When one has access to Flare Revolvers the MAC-10 is primarily attractive for its efficiency on lower difficulties (where it is phenomenal for decapping trash) or to use WITH the Flare Revolver (e.g. Trenchgun, MAC-10, single Flare Revolver).
The Flare Revolver deals 125 initial damage compared to 35 for the MAC-10. Additionally the flare has a small AoE, lightly damaging and igniting nearby enemies (only 25 initial damage for the AoE). The damage over time is higher for the Flare Revolver and unlike the MAC-10, Flare DoT scales with perk level. It appears that at level 6 a flare does almost twice the DoT of a MAC-10.
So what does this extra damage mean in practice? On Suicidal/HoE the MAC-10 loses the ability to decap weak specimens in one headshot; the Flare Revolver always can. In fact the Flare Revolver can always kill a crawler on bodyshot, can kill Sirens on bodyshot with sufficient level (lvl 4 needed for 6 man HoE), and with sufficient level will kill clots on the first tick of afterburn damage (or outright on lower difficulties). Gorefasts are far less prone to bodyshotting; they'll die after one hit but it'll take a long time for them to burn to death. However, a single headshot will instantly kill a Gorefast (possible level requirement for higher difficulties); this can be difficult particularly with duals, but when the Gorefast is in your face it's relatively simple even firing from the hip. And starting at level 5 the Firebug gains the ability to flinch lock Scrakes with Flare Revovlers.
The reason that dual Flare Revolvers + Husk Fireball Launcher is probably the best overall loadout is because the Flare Revolvers are plenty effective against trash. If the team is heavily relying on the Firebug for trash control it's still better to have Flamethrower or Trenchgun. But this usually isn't an issue; if I can excel against trash with the Flare Revolvers alone why not take the Husk Fireball Launcher and be ready to help with Scrake/FP when needed? Scrakes may not be the Firebug's job but when the designated Scrake killer(s) are occupied, sucking, or dying it's really nice to have someone ready to step up. In almost all cases the reason I forgo the Husk Fireball Launcher is price - if going cheap and feeding cash to my teammates increases our chance of success I'm happy to do so.