How can I earn Foxfire Candle Stubs quickly?
The most efficient way: the right storylet
Find a storylet available to you that rewards you in Foxfire Candle Stubs. Storylets reward you in direct proportion to the minimum main quality required to unlock them, so find the hardest one that you can succeed in on a fairly-regular basis. (For example: a storylet that requires Watchful 20 might reward you with 20 Foxfire Candle Stubs, but a storylet that requires Watchful 50 might reward you with 50 Foxfire Candle Stubs.)
Alas, I can't be more precise than that: most of Fallen London will be unavailable to you at any given moment, because one of your four main qualities will be too high or too low. Storylets are only available for a particular window: if your Watchful is too low, you don't get to see, say, Search For A Missing Rubbery Man in Ladybones Road. Similarly, it will disappear again once your Watchful gets too high.
In general, storylets involving Knife-And-Candle players, New Newgate Prison, and the Church will get you Foxfire Candle Stubs.
The fastest way: Money, Dear Boy
If you really need a lot of candles fast, then the simplest answer is to sell things, and buy Foxfire Candle Stubs at 3 pence apiece from Merrigans Exchange in the Echo Bazaar. (It's the "Bazaar" tab.)
Anything in the "Goods" category is ideal to sell, because those items can't be converted into other things and doesn't have any use except to be spent. Rostygold, Moon-Pearls, Nevercold Brass Slivers, and other such Goods have uses in storylets, but can't be up-converted into other things. Chances are really high that you have far too many of several different types of Goods, so sell off the excess: remember how many you used the last time a storylet required that Good, and sell off all of that Good except for a small reserve. Repeat for the rest of your Goods, until you have enough Echoes to buy what you need from the Bazaar. If you can't or don't want to sell Goods, then sell the base-level up-convertible things, like Whispered Secrets, Jade Fragments, and whatever else that you've ended up with far too much of.
The main idea here is that you will almost never have 1000 of the thing you need, but you'll almost always have 2000 various other things that you aren't using terribly much at the moment. Since you can't really predict what you'll need next, it's a better use of your time to buy what you need now, rather than to save up forever in the hopes that you'll eventually find a storylet that requires those 10,000 Whispered Secrets that are cluttering up your cupboards.
Actually, for a player in 40-80 range1, best spot would be
Attend a service at the Melancholy Curate's church
storylet, Unlocked with Intrigue: The curate and his sister 1, Investigating... 3, Persuasive 33, and Locked with Investigating... 9 - since it
- gives you a guaranteed 37 FCS,
- is required anyway for progression of the 'Curate and his sister' storyline,
- gets
straightforward (100%)@64 PER, - allows you to produce FCS in mass amounts (as required by Bishop's progression).
There are e.g. some cards giving FCS and some other FCS-rewarding storylets, but they're usually a one-shot FCS reward only, grossly out of your stat range, and note that it's rather impossible to farm anything in mass quantities in a reasonable amount of time with cards only, due to their randomness.
Also, note that IMO saying "I can't be more precise than that: most of Fallen London will be unavailable to you at any given moment, because one of your four main qualities will be too high or too low." is wrong - you can easily tune up/down your qualities by about -30 to +20 by using proper items without any high-end stuff - getting a couple of +5 items works wonders here, as well as using Talkative Rattus Feber (-25 to all) etc.
Note that farming money is actually less effective here, since it'd require to get > 1 Echo / action to give you higher output of FCS.
1 "Heart's Desire" quest begins in that range