Perform the suggestions mentioned in the following articles:

• Firefox uses too much memory or CPU resources - How to fix
• http://kb.mozillazine.org/Reducing_memory_usage_-_Firefox
• Firefox uses too much memory or CPU resources - How to fix
• http://kb.mozillazine.org/Firefox_CPU_usage

Check and tell if its working.

Checked those articles and they're not helping. Do take a look at the screencast. Disabled most of the plugins and all extensions. No extra theme either.

Just opening up Firefox alone and leave it for 5 minutes will still take 900 MB.

If you use about:memory you will see exactly what is using the memory, and have buttons to reduce memory use. To do that all you need do is type about:memory into the location bar and hit return.

If you open Firefox in safe mode on a blank tab it will probably not use anything like 900MB try it and see what happens. Then try more normal use a step at a time and compare the results.

Create a new profile as a test to check if your current profile is causing the problems.

See "Basic Troubleshooting: Make a new profile":

• https://support.mozilla.org/kb/Basic+Troubleshooting#w_8-make-a-new-profile

If the new profile works then you can transfer some files from the old profile to that new profile, but be careful not to copy corrupted files.

See:

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox