I am having the same problem. I live on the Mass/Rhode Island border. I have 1-1/2" foam insulation under my five gallon pail pots (drilled for drainage). My trees are approximately five feet tall (from top of pail to top of tree). The pail/pots are white. For such a supposedly easy plant to grow, I am at my wits end. I have six trees of which two are having this same problem. I fertilize in the spring with 1 part super phosphate, 1 part bone meal and 1 part garden lime. Then I lightly fertilize with tomato fertilizer every twenty one days. I use a moisture meter probe so I never let the wilt factor go below 2.5. All instructions say this is a full sun plant. My next door neighbor has his four foot tree in a partially shaded area and he does not have this problem.
I also read that these pots should be set in the ground. I would assume this is to keep the temperature of the roots more cool and constant.
DESPERATE - PLEASE HELP

pots too hot!!!!!!!! mulch the top and cover the pot with aluminum foil or better yet bury the pot. black plastic in the sun is a death wish for fig trees. also the older your tree gets the more root bound it will become so upgrading you pot wouldn't hurt. once your tree gets so large ie 1ft per 1 gallon you'll need to root prune in the winter unless you keep going larger and larger on the pot then health becomes a factor are you physical able to move a 200 or 300 lb tree? the more root bound the tree is the more it will suffer from heat damage as the pot heats up. It's true figs love heat but the roots seek out cold dark damp places like under a house or any other structures. fertilizer won't help, and you shouldn't even use much nitrogen if any if trying to leave the plant in a pot it's whole life. a granulated bulb food works but anything with low N and high P & K works well for potted plants. Organics in pots just don't work well and I'm an organic nut. to move product yes it's possible but to sustain a plant in a pot over a long period forget about it. it's too hard to regulate the temps of the pots above ground to sustain microbial life in the pots. So in pots commercial nutes are the way to go. here in SC I have plants in 1-5 gallon pots and in ground the potted plants will be put in ground, traded, or sold. I have no intentions on growing figs in pots because of the hassle that comes with older larger trees. there are varieties that will grow in ground in zone 6b with limited protection you should try some of these varieties in ground unless you are ready to fight an uphill battle your whole life because the tree will outlive you if you keep it healthy. good luck

Thanks for the tips! I will get some aluminum foil around the pot. This variety of Fig will die in Zone 6a hence I potted it. Yes its going to be a pain dragging it in and out of the garage but I have no choice.