Follow these steps to check what is causing the abnormal memory usage:

• Check your Extensions and Themes.
• Check the Plug-ins.
• Clear Download History.
• Restart Firefox Periodically.

1. Check your Extensions and Themes: 

If you have a number of Extensions and Themes installed on your Firefox, try to minimize them, remove those which are not in use or you don'g really need.  You can check it by opening Firefox in safe mode.  Here's how you do it:

1. Close your Firefox browser.
2. From the desktop, click Start > Run.
   
   
3. Key in, firefox -safe-mode.
   
   
4. You'll see a new window called "Firefox Safe Mode".
   
   
   • Just select the options displayed and if you click "Make Changes and Restart", Firefox applies changes and restarts in normal mode.
   • If you choose to select "Continue in Safe Mode" no settings will be applied.
   • If you choose to "Exit" the browser will just exit without saving any settings.

2. Check the Plug-ins:

Firefox Plug-ins consume memory when in use.  It is always important to check and update your plug-ins to the latest version. It is also good to uninstall all those plug-ins that are not in use.

3. Clear Download History:

Keeping the Download History can cause extensive usage of memory and this can slow down or freeze Firefox. 

1. Delete the "downloads.rdf" file to clear the download history.
2. You need to make a little change to the browser setting.  Here's what you do:
3. Open Firefox browser.
4. From the menu bar, click Tools -> Options.
   
   
5. Click Privacy.  Un-check the "Remember What I've downloaded" check box.
   
   

4. Restart Firefox Periodically:

There might be a constant increase in the memory usage if you are using Firefox for a long period of time or handling multiple sessions.

1. Consider restarting your browser periodically so that it can maintain a constant memory usage.  
2. Download and Install Session Manager Extension, which would save the current text data, history and cookies.  It allows you to maintain the opened web pages when Firefox is closed.

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