Yes you can use it, no it is not the same. You need to look up the recommended loads/grains for your rifle for FFF powder vs. FF powder. FFF Uses LESS than FF. If you cannot find it on line then contact the manufacturer and they will usually send a free Owner's Manual.
I have a book about loading and shooting muzzleloaders. It is an old booki and doesn't have any info on inline ignitions. However the only difference between percussion cap shooting and inine shooting is the primer is different and the hole where the primer fire gets into the charge is in a different place. My book sez ffg for 50 cal and up. fffg for rifles less than 50 cal and ffffg in flash pans. Pyrodex is supposed to be the equivelent of BP. Use that advice, sell me ur fffg cheap and u can buy more ffg for ur rifle.
Pan, a 50 caliber is the mid point between the two powders. Anything smaller than 50 uses FFFg. Anything larger, uses FFg. The 50, itself, can use either one. That depends on the actual rifle. I have tried both powders in both my rifles and although the difference is almost negligible, they like FFFg better. In a pinch, I have also primed the pan on my flintlock with FFFg, with no problems.
Good luck with your rifle, and keep us posted on your results.
90 grains of fffg in a54 cal is like putting 120 grains of powder in a area ment for 90 grains of ffg.
Think of it like a bow and arrow. You are shooting a 120 pound bow pull using a arrow with a 90 pound spine strength
No real reason you can not use FFFG in a 54. I have one 54 that shoots best with FFFG and likewise a 50 that shoots best with FFG. Point is you have to spend the time developing a load, reading patches etc. There are only a few really hard and fast rules about these old guns, most everything else is more a general guideline and it is up to you to figure out what works best in your situation.
From what I've been told by the old-timer BP shooters I know, FFFg will build up more pressure than FFG, so if you use it in a big-caliber rifle you need to use considerably less of it than you would FFG for safety's sake. Never tried shooting FFFg in my .50 or my .54 myself, so I dunno.