I've not noticed any specific issues with Firefox slowing down.
I had a look at some of the sites which I look after and the distribution differs depending on what type of site it is.
For sites which would appeal to the general public then it is around 50% IE, 25% Firefox and 10% Chrome.
For sites which appeal to more technically orientated people then the percentage of IE and Firefox is around 35% each and Chrome is around 15%.
In both cases, Chrome has taken about the same % of users from IE and Firefox over stats from last year.