firefox start error - Couldn't load XPCOM

firefox start error - Couldn't load XPCOM

Check ubuntu is 32 bit or 64 bit and firefox is 32 bit or 64 bit.
Generally you get this when you install 32 bit firefox on 64 bit ubuntu.

Check your firefox is 32 bit or 64 bit, run in firefox
about:

If you get
Build identifier: Mozilla/5.0 (X11; Linux i686; rv:36.0) Gecko/20100101 Firefox/36.0
Note: i686, i386, i586 are 32-bit builds.

and if you get
Build identifier: Mozilla/5.0 (X11; Linux x86_64; rv:36.0) Gecko/20100101 Firefox/36.0
Note: x86_64 is 64-bit build.

Download 64 bit firefox from here.
Download 32 bit firefox from here.