Dont try downloading any vids (your probably not) on a firestick. it doesnt have enough memory.
Try to keep the number of addons you install to only what you use to save space.
As @RavRob said, in the Maintenance Tools section of Indigo you can see how much memory the cache, thumbnails, and packages are using.
all three can be cleared (deleted) with out harming anything.
And like @gujal said the thumbnails use the most