The security header is not valid

Regarding Paypal API credentials, here's a summary to avoid confusion:

* If you're using SANDBOX mode, you need to use a SANDBOX account API credentials.
* If you're using LIVE mode, you need to use a LIVE account API credentials.

If you use a LIVE account to test in Paypal's SANDBOX environment, Paypal will respond with "The security header is not valid" and vice versa.

There must be a good reason why the returned message is so vague.

Delete individual URL from Firefox's address text input

Shift + Delete

Simple right ?

You can also do this more than once. For example, while typing "www.c" on Firefox's address text input, it will display other URI starting with those characters. Here, you can use the arrow key to highlight the URL you want to remove and press "Shift + Delete". Repeat as necessary for other unwanted URI.

It was unusually difficult to locate this feature by navigating Firefox's menus, so I Googled. The result is posted here just to spread the word.

Linux fc tips

fc is a command in Linux which I can't seem to remember no matter how many distros I hopped onto.

So I post this as a reminder, as well as for the benefit of anyone who despise reading man pages :)

To list your executed command history:

fc -l

The above will lists the commands with line numbers on the left like so:

492  unrar x some.part1.rar
493  sync;exit
494  python editor_test.py 
495  cd Documents/python_editor/
496  vi test.py
497  python test.py

To execute command history number 495 above:

fc -s 495

To edit it before executing however, use the one below.
After you exit the editor, the command will be executed:

fc 495

If you would like to execute the last command with the word 'mailer' in it:

!?mailer?