I am planning to purchase Maxxis Larsen TT 26 x 2.0 tire for my Firefox Cyclone D 26 bicycle.
My bicycle currently has 26 x 2.125 tire. Just wanted to know if the above Maxxis tire will fit or not.
Yes. It will work fine.
Judging by the description of the rims, it is likely they are Weinmann 519 or Zach 19s, or something very similar. Most of these generic double wall rims fall right around 19mm for inner width. This means they will comfortably hold a tire from 1.20" to 2.50". Many if not most big box store bikes use this size for that exact reason. There are a lot of readily available tire options and they are one of the more mass produced sizes so the cost of purchasing the rims as well as tires is cheap.
It's impossible to be 100% sure as the link you provided does not provide the internal width of your rims.
However since you are going to a smaller tyre there should be no problems with frame clearance which makes it extremely likely the new 2.0 tyre will be fine