What would happen to the mice if all of the foxes left this ecosystem?

In an ecosystem, there are food chains and a food web, which is composed of interconnected food chains. Each organism, in an ecosystem, is part of a food chain and the food web. If any change takes place at any trophic level in a food chain, the results are significant, while those in a food web are relatively mild. 

For example, mice are common prey for foxes. If all the foxes are removed from the ecosystem, mice will have less predators. Remember that foxes are not the only predators of mice and in a food web, all the prey and predator are connected to each other. Thus, the removal of foxes will reduce the total predator count, not reduce it to 0. A reduction in the predator population will cause an increase in the mice population. More mice will mean more food for other predators of mice (say snakes) and thus the snake population will also rise. An increase in the mice population will also cause a reduction in the food of mice, since more mice will eat more food. Thus, for a brief duration, the food of mice will be under stress, while the population of mice and other predators of mice will increase. However, a new equilibrium will soon be established between mice and their other predators and the ecosystem will continue to function without major modifications.

Hope this helps.