I kept close track of what happened when an enemy Fire Bolt hit me in a recent battle.
- The initial hit, the d6 rolled 2 fire damage. All the remaining dice are d4's.
- The hit inflicted burning, another 4 damage.
- The surface underneath me rolled a 1, but we're already up to 1d6 and 2d4.
- Finally, my turn again.
- I lost the burning condition, but the surface beneath me was still on fire.
- Walking off the surface inflicted burning on me again and did another 2 damage.
- When the burning status rolled around again, another 3 damage, then the status expired.
- The damage total from the single Fire Bolt was 12, but the total dice rolled were 1d6 and 4d4. Total range of damage is 5-22.
Some people might argue that the last 2 d4's were avoidable, if you use jump, but even if you want to keep the surface, it should not tick not tick twice. It's a cantrip.
****
Dipping weapons in a fire source is fucking freely available if you actually play the game, it's called a candle, and can be freely dropped and lit.
Thinking about it for a moment, this defense is ridiculous. Are you seriously going to carry around candles all the time, and spend all those extra seconds in every battle to put a little bit of fire on your weapon?
Maybe you are, but you are comparing having to tediously drop and dip candles in every battle with the simplicity of select cantrip, click on enemy. You're spending a lot more effort to get that flaming weapon than the mage is to cast Firebolt.