If the fire button is getting hot it would seem to indicate a higher than normal resistance in the fire button. That is a problem in lower quality switches and can be made worse by foreign materials, water, e-liquids, getting into the contacts.
If you're experiencing a burned taste it would seem to indicate the coil is running too hot or the wick is not supplying enough liquid to keep the wick cool enough not to burn the liquid that is delivered. Maybe try a higher ohm coil or try a different wick material, like rayon or cotton rather than silica.
In terms of batteries, I tend to stick with Panasonic, LG or AW and stay away from the other brands. Check the reviews and any independent tests done on the batteries that you do have.