Why do my internet browsers stop working? I have a 13″ mid 2009 MacBook Pro. 

For a long time now my browsers just decide to stop working. The 1st occasion Safari kept crashing every 10 minutes so I downloaded Firefox but after a while the same thing kept happening.

I thought it was something to do with Flash so I downloaded a newer version of Flash but that didn't solve anything. I then downloaded Chrome which was fine for a while but now from time to time any browser I use just stops working. I find myself either having to switch my wifi off and on and restart which ever browser I'm using or if that doesn't work, when Safari stops working I switch to Chrome and vice versa.
The first step in narrowing this down is to look at the crash log and the event log in Console.app (under /Applications/Utilities) to look for relevant error messages. 

Usually it is a dns related error, but if that doesn't work, then run the network diagnostics to make sure it isn't just a web browser issue.