So basically you want mages with elemental powers being useful in modern society, and your issue stems from fact that fire is mostly used for destruction, and has in many applications been replaced by more controlled alternatives.
This instantly gives us first idea how to use such powers for benefit of the mankind.
Fire is, and has always been a powerful destructive force. When controlled, it can be harnessed to provide heat and power, when uncontrolled, it can devastate forests and cities alike.
Ability to control fire would be tremendous asset for firefighters. Depending on how fine the control is, risk of fire spreading between buildings could be eliminated (this a very real problem, when fire is bad enough, job of firefighters is not to save burning building, but to prevent fire from devastating nearby buildings) or maybe even fire itself could be extinguished.
Forest fires would still pose massive logistical problem due to sheer size, but would be much easier to tackle. Fine enough control, and your mages could help with preventative controlled forest fires. Including making sure those controlled fires don't spiral out of control.
But this doesn't limit us just to preventing or extinguishing fire.
At the most basic level, in thermal power plants electricity comes from turbines powered by steam. It doesn't matter what fuels the "furnace" - coal, gas or uranium, principle is the same - "furnace" heats water and makes steam, steam drives turbines. Same with ships. Yes, in a way, nuclear submarine and nuclear aircraft carrier are steam ships.
If your mages can create fire which doesn't need fuel and is easily controlled and maintained, they could easily break energy economy, solving pretty much all needs if their abilities are powerful or plentiful enough.
Basically, all the efficiency and cleanliness of nuclear power, without any of the risks. What's not to love?
Disclaimer: In case anyone questions: yes, nuclear power is clean, safe and reliable power source, beating every other source on at least 2 of those criteria. Single broken hydroplant dam kills more people than all nuclear incidents combined. Fossil fuels fuel greenhouse effect which may literally kill us all. Wind turbines don't work without wind - unreliable. Solar panels barely work in cloudy weather or in winter - unreliable. Geothermal comes close, but availability varies based on local geology so predictable but not widely reliable.
Neither of those is a very hands-on approach, like missionaries handing out clean water or blankets, but I assure you, that both would be very beneficial for modern (and in times before modern even more so) people, poor or rich alike.