Mitch4 sends this in, with a brief comment: “The signs about briefs probably mean legal briefs rather than underwear briefs, since the tables seat these professionals with briefcases. But either way, what does it have to do with “firmers”, whatever those are? I mean, you don’t need to write a brief to establish a business firm.”
Comics I Don't Understand 
Farmers grow produce. They sell surplus produce at farmers’ markets. These guys have law firms, which write briefs. They sell surplus briefs at firmers’ markets.
It’s true that not all law firms write briefs! Likewise many farmers don’t grow produce. That’s not important to the joke. I thought it was a little clever, if not exactly funny.
@Andrew, all good points, but it doesn’t explain the word “firmers”. “Firmer” could be the comparative degree of the existing adjective “firm”; or an agentive formed from a nonce verb “[to] firm”. But to my ears neither really fits.
Farms farm. Firmers work at a firm. It’s agentive.
I don’t know about ‘firmers’, but I would like to know why baby Dennis the Menace is in the shopping cart.
It’s an unusual use of firmer to refer to someone who works at a firm. More standardly:
“firmers
Plural form of firmer
adjective
Comparative form of firm: more firm.
noun
A physical exercise or cosmetic product intended to firm part of the body.
A thin chisel with a tang to enter the handle instead of a socket for receiving it.”
So firmers might be something like a box of girdles.
That might actually be funnier: a Firmers Market with booths for girdles, those strange looking ab exercisers, botox, etc.
It’s not Dennis the Menace. Dennis the Menace always wears a striped shirt. (The other unrelated Dennis the Menace, in England, also always wears a striped shirt.)