Firefox Problem Taking up all CPU

My firefox web browser is constantly constantly taking up alll of my CPU and making it impossible to perform other tasks at the same time. Also my internet is very unusually slow. I have wireless and 2 other people use this on there laptops (mine is a desktop) one of them is basically always on the internet.
which i realize splits the speed in half right? firefox has just recently began closing down more often with "experiancing problems…sorry for the inconveniance… "blah blah…
well i have a feeling that firefox has some bug or something here because idk if it is supposed to take up all my CPU all the time or what. its just not right. something isn’t adding up here. we have plenty of RAM and i don’t know what could possibly be the problem here

responses are greatly appreciated

Which is your operational system?
Which Firefox version are you using?
Any special extension installed?
Which was your old antivirus (before avast)? How did you uninstall it?
Any other security program that could interfere?