This is a quick guide to getting guaranteed good bars in 5-4 as small mario or small fire mario.
What you're going to want to do is look at the rotation of the very first bar in the corridor:
If that bar is rotated to where it's pointing away from mario, then you have good bars and can just keep on going with no slowdown!
However, if the bar is rotated to where it points straight up or toward mario you have a bad bar. The more tilted it is toward mario, the worse bars you will get.
Good news though, to manipulate the bars all you have to do is notice that the first bar is pointing at a bad angle, then release B after landing in the corridor for about half a second while the first bar is still on the screen.
The longer you release B for, the better the rest of the bars will be, so if you want to be extra certain that the bars are good, just release for a little longer.
By releasing for a shorter amount of time you can turn a bad bar into a medium difficulty bar while only losing 4 frames or all the way to a good bar at a cost of 7 frames.
Even if you do release B on a framerule with good bars they won't become bad bars or anything like that.
So it just so happens that the first firebar in the corridor, the last firebar in the corridor and the firebar after the elevator section all load into the same enemy slot.
Because firebars actually don't reset their rotation when they spawn, they will start with the rotation that the last firebar in that enemy slot was at when it despawned. So by slowing down you let the first firebar spin for longer, which puts it in a safer spot for the next two bars!
The reason the firebars are at different angles per framerule in the first place is actually the podoboos at the start of the level. The podoboos write data to the rotation angle memory that the firebars use, so the very first podoboo actually changes the rotation angle of the first firebar in the corridor to one of 4 different values, 11, 22, 33, or 2000-some degrees.. If you don't get the 33 degree angle from the first podoboo, your firebars will be suboptimal!
Hopefully this will help you not lose as much time to bad firebars in 5-4, and not have to be as concerned about bad 5-4 framerules. :)