You are probably using up the memory and the browser crashes. Use fewer open tabs. Each tab takes up memory.
Which version of Firefox (try help->about)?
Which operating system?
What plugins and extensions have you installed in Firefox?
If you do help->Restart with addons disabled, do you get a better experience?
Note: if you're running various ad-blockers and script blockers, then only visit sites you know and trust when running with addons disabled.