Firefox sometimes uses more memory (RAM) than it should. This can make Firefox slower, and in extreme cases, it can even make Firefox crash. This article describes how to make Firefox use less memory.
The latest Firefox version includes improvements about memory usage. Update to the latest version.
Extensions and themes can cause Firefox to use more memory than it normally would.
To determine if an extension or theme is causing Firefox to use too much memory, start Firefox in Safe Mode and observe its memory usage. In Safe Mode, extensions and themes are disabled, so if you notice a significant improvement, you can try disabling or uninstalling extensions.
Many web pages have content you don't really need that uses memory to display (see the plugin section below). Some extensions allow you to block such unnecessary content:
Plugins to display special types of content can consume large amounts of memory, especially older versions.
To check to see if you have the latest versions of all your plugins, go to our Plugin Check page.
You can test to see whether one of your plugins is causing Firefox to use too much memory by selectively disabling some of them:
Click the menu button and choose Add-ons. The Add-ons Manager tab will open.
After disabling some of your plugins, close and restart Firefox, and observe its memory usage. If you don't see an improvement, you can enable those plugins again and try with a different set.
If you do see an improvement in Firefox's memory usage after disabling a certain plugin, you may leave it disabled. If you can't because its use is widespread on Internet, try to find an alternative lighter plugin.
For certain plugins that play videos such as Flash, the content rendering in full screen can be accelerated by hardware. It eases memory usage in case of a dedicated graphic card memory.
Firefox hardware acceleration eases memory usage in case of a dedicated graphic card memory.
Firefox's memory usage may increase if it's left open for long periods of time. A workaround for this is to periodically restart Firefox. You can configure Firefox to save your tabs and windows so that when you start it again, you can start where you left off. See Restore previous session - Configure when Firefox shows your most recent tabs and windows for details.
Each tab requires Firefox to store a web page in memory. If you frequently have more than 100 tabs open, consider using a more lightweight mechanism to keep track of pages to read and things to do, such as:
Having many applications running simultaneously may cause your computer to run slowly and other applications to as well. By closing down some of the unnecessary applications, memory usage will be reduced.
If you exhausted all tips in the previous sections and your memory usage is still close to the maximum, maybe it's time for you to add more memory to your computer. RAM is cheap and will provide a huge performance boost.
Based on information from Reducing memory usage - Firefox (mozillaZine KB)