Mathieson's operated from about 1822 to about 1966, in various places; Glasgow pretty well throughout their existence, but also at various times in Edinburgh, Dundee and Liverpool. Consequently, the name doesn't help us date the chisel at all! It's also true that a lot of tools were made in Sheffield and marked with the names of other tool makers or retailers, so there's no absolute guarantee that Mathieson's actually made it.
In 'The Tool Chest of Benjamin Seaton' there's a series of sketches of chisel shapes from early 18th century to late 19th. The shoulder shape of the chisel in question corresponds to shape in the sketch labelled as early 19th century. It so happens that at that time, chisels were made with larger bolsters than later ones, because ferrules were not used until about 1840/1850 (no reliable brass tube available until then). The bolster on the chisel in question looks a little larger than ones most of us are used to, but not greatly so. Thus, the chisel MAY be somewhere between - perhaps mid 19th century.
The blades of 19th century chisels tended to be thinner than 20th century ones. (They also tended to taper in thickness as the OP noted, and in width, being wider at the cutting edge than the shoulder by about 1/32" to 1/16". The sides were often not dead square to the top and bottom faces, but had a slight bevel of about 5 degrees or a bit less.) It seems that bevelled-edged chisels did not appear in the catalogues until about 1890 to 1900, so until then, the thin 'firmers' were used for what we now regard as 'bevel-edged' duties - a thin chisel can get into places a fatter one can't. Clearly, they wouldn't tolerate tasks that required a levering-out of waste, but then neither would a modern bevel-edged chisel; the 19th century woodworker would use a wrought-iron socket chisel with a laminated edge for rougher work. However, the length of this one suggests a shortened paring chisel. It's a bit over-length for a firmer.
A failing of the 19th century plain carbon 'cast' steel is that it needed a very fast quench to achieve full hardness (using water rather than oil). The result of the fast quench was a lot of internal stresses in the hardened chisels, some of which could be released as the surfaces were ground off. That could mean that chisels distorted rather more than we now regard as normal; the grinder sometimes selected the concave-in-length face for the cutting edge, grinding the bevel on the convex face.
If the chisel is of mid 19th century steel, it will in all probability sharpen easily, and take a really keen edge. It could well be worth showing it to your finest polishing stone - it could be a really good one.