"Accessing your about:config page
In your Firefox, type about:config in the address bar. You will be
shown a warning page. Click the "I'll be careful, I promise!"
button to proceed.
"On the main page, you will see a long list of configuration
entries. Enter the name of the key you want to update in the
"Filter" field. The list will narrow to only the entries that match
your keyword as you type.
"To modify the value, simply double click on the entry value
field and update the entry. That's all! Isn't that simple? Now,
let's get to the tweaking."