If you are like me, you usually have many tabs open at once and every so often your browser will slow to a crawl. Identifying the culprit is usually based on a lucky guess, but browsers have some in-built tools to help you.
In Firefox, type about:performance into the address bar.
In Chrome, go to the menu to the right off the address bar (3 vertical dots), select More Tools and then Task Manager.
Both of these will show you which tabs are using the most resources and allow you to close the tab from within them.