There’s not much difference really, just the shape of it. There're both good at chopping out a mortice but of course a mortice chisel does a better job. A Firmer chisel is a bit of a general-purpose chisel.
Hope this clear things up but I'm sure someone like Jester can give a better answer
A traditional firmer chisel is fairly thin, certainly no thicker than the blades found on most modern bevel edge chisels. Some of my old ones are very thin indeed by todays standards, more like paring chisels.
Next up are the registered mortise chisels. Thicker blade again, handle generally made from ash or hickory with a leather piece inserted betweeen the tang of the chisel and the handle - they were intended for use by ships joiners and for general rough work and the strengthening ring allowed them to be driven by a mallet or hammer.
Thicker bladed yet are the cabinetmaker's sash mortise chisels - a sort of cross between a heavy mortise chisel and a registered chisel with the leather shock absorber. Because there were intended to be driven in the main by hand (or at ,most with gentle taps from a mallet) they don't have any strengthening ring at the top and frequently come with handles in boxwood. They are limited in size range from about 1/8 in to 1in wide because cabinetmakers normally don't go in for girt great motises and tenons. Charley's example is a continental-pattern mortise chisel - like a sash mortise in weight, but with the hoop seen on an English-pattern registered mortise chisel
The biggest and heaviest of them all are the haevy mortise chisels. They go up from about 1/8in the 1-1/2in and my set have blades which are nearly an inch deep at the tang. Once again leather shock-absorbing washer, but this time home-made handles (they were frequently supplied without handles). They are designed to undertake heavy and deep mortising work and are driven by a mallet.
The true mortise chisels all share a common characteristic - the edge of the blade between the ground face and the front edge is always rounded over. This allows them to be used to pry chips out in the mortise.
You may also come across what are referred to as swan neck mortise chisels. Their peculiar swan-neck curve blades allow you to clean out the chips at the bottom of deep mortises.
Firmers are not really designed for heavy mortising, the deeper section of a true mortise chisel will alway cut squarer with less tendency to wander
Heh heh, I've answered this one at "the other place", and Scrit and I almost agree on some of it too! Chisels are a nightmare to classify; not only were there so many different varieties, but what is meant by a certain name may have been changed over time. I admit I'd forgotten about the fine nuances between the sash mortice and the "registered" type. I think the main distinguishing feature of the latter is the leather washer, as Scrit mentions, and the longer ferrule which is solid at one end with a square hole for the tang. Not that it matters much... :wink: The easiest rule of thumb is that mortice chisels have a thicker blade.