[nighthawk (OP)]

I know that fluorescent lights are usually not good when it gets too cold. They can't glow as bright as when they are at room temperature. But what happens with power consumption? They are dimmer because the density of mercury atoms drops, and there are less collisions with electrons. But the number of electrons should be the same, so the power consumption should also be the same. Am I right?

[lyner]

Why should the number of electrons (you mean per second?) be the same? One would have to measure the current to make sure.
It is the electrons which move and the mercury ions stay relatively still. The heaters at each end produce the initial population of free electrons by thermionic emission. After the tube has 'struck', there is enough ionisation to keep the process going.

If there are fewer mercury atoms floating around (vaporised) then there will be fewer available to be ionised.
I  should imagine that the ion density is lower in a cold tube and the 'resistance' would increase, reducing the current and, thus, the power consumed.
The presence of the series ballast makes things more complicated regarding the eventual voltage which establishes across the tube.