As far as firefox is concerned it is a single process , no matter how many tabs you open , still it runs as a single process in your computer , so if you open more number of tabs you can expect your fire fox to work a bit slow , this has not connection with your internet connection , whether it is broadband or a dialup , moreover the main disadvantage here is that if there is an error or some problem in anyone of the tab , then they whole process is stopped which leads to the shut down of firefox.
Whereas in Chrome , the condition is diffrent , each tab runs as a seperate process , so you neednt worry

Chrome RoCks !