Sunlight seems to be the best cure for thin weak stems. More and better sunlight is what tripled my dracaena marginata's previously-willowy stem since last September, and it's the first advice I see given to other people with thin-stemmed FLFs.

Figs just adore light. Honestly, your tree needs as many photons as you can provide, and would be very happy sitting in broad daylight all day (after allowing a little time to adapt to strong direct sunlight). But if you can't get it outdoors, put that plant right in front of and very close to a south or southwest facing window with nothing but glass between it and the sun. Leave the support on the plant until it's strong enough to stand on its own.

Or, if all your windows face east like mine do, position the tree where it will get the most morning sunlight and supplement the light the rest of the day with 100W bright white LEDs or CFLs when it's indoors. I use a cheap 24 hour timer so I don't have to remember to turn the lights on and off.

I hope that helps!

Thanks Litterbuggy!