Wind speed over which a Canadair fire fighting plane cannot safely fly?
The problem is not the wind speed per se, but the combination of factors.

Let's start from the data; this is the METAR for Athens on the 17th of July, the day of the fires:


  METAR LGAV 171220Z 03026G38KT CAVOK 32/10 Q1016 NOSIG=


I have highlighted the wind speed data. It says that the wind at Athens airport was on average at 26 kts, with gusts up to 38 kts


26 kts of wind at high altitude would not be a problem for a CL-215
26 kts of wind near ground (to drop the water it has to do low passes) and at low speed (it has to go as slow as possible to be precise) can be highly dangerous, as proximity with the ground increases turbulence. Let's also consider the mountainous terrain around Athen (and the fires were on the side of a mountain), windshear and turbulence were probably quite strong. (See also this question: When crossing a mountain ridge at low height above terrain what consideration is given to turbulence? )
26 kts was the average wind speed, gusts were up to 38 kts, increasing the risks of such operations.


In addition to the above, let's remember that these aircrafts have to fly over fires, that will create additional turbulence through the rising hot air column that they generate. (thanks to David Richerby)

I do not think that a hard limit for such operation exists, as said above is the combination of windspeed, terrain configuration and mission that determines if an aircraft can perform it or not.