Hi Brian,
Am I right in thinking he's a "fireman" on the 1891 Census, I've been unable to find him on there ?.
However I could find him as simply John Russell on the 1881 as a "general labourer" and his 1887 marriage PR says he's a "collier". I very much doubt any "fireman" in those days in the Dean would be employed as a fulltime professional firefighter; indeed 80odd years later the local brigades were still volunteers aka "retained", I know Cinderford was, they probably still are.
The term most usually described the stoker of a steam engine, which seems likely given John's previous jobs, that engine could have been a stationary winding engine at the pit head, or maybe a smaller engine driving the ventilation fans at the bigger/most modern pits. Alternatively he may have been employed maintaining the fires that were burnt within ventilation outlet shafts to ensure a draught of fresh air thro' the underground workings of smaller/older pits.

As Mike suggested the best source may well be a newspaper report. I've tried various searches on the excellent Welsh Newspapers site looking for "Dean Forest miner" etc etc, filtered to just the 1890s, but sadly no luck in this case. It does carry quite a few such accidents(usually reporting on the Inquest), not unusual events then sadly, hopefully you'll have more luck than me.
http://welshnewspapers.llgc.org.uk/en/home

I couldn't find anything on the BNA site either ?. However please note these sites don't carry the Forest's own "Mercury" archives, to search them means visiting Cinderford library if you're able.

Finally this database includes a great many, but not all, of the mining deaths across the UK; again I couldnt find your John Russell listed.
http://www.cmhrc.co.uk/site/disasters/index.html

Sorry couldnt be more help, but feel free to search the site's yourself, they often give surprisingly different "hits" for just a little tweeking of the search words.

Jeff