[nhebb]

Go to about:memory then click the "Minimize memory usage" button.

P.S. If anyone knows how to add that button to the FF toolbar, I would consider you a minor deity for at least a whole day.

[MikeCapone]

Interesting. What does pressing that button actually do?

[lambda_cube]

If you hover that button with your mouse pointer you can read the following explanation:

Send three "heap-minimize" notifications in a row. Each notification triggers a global garbage collection followed by a cycle collection, and causes the process to reduce memory usage in other ways, e.g. by flushing various caches.