You used to be able to add formatting to posts by creating a Note type post, but Facebook have now removed that functionality, so read on for other options.
If you want to be able to use formatting in a normal status update or a comment on someone's timeline you can use a tool that converts your text into different unicode fonts, depending on which formatting type you require, which you can copy and paste into the status or comment box.
Here's a list of font changing tools you can use :
http://fsymbols.com/generators/
https://mothereff.in/twitalics
http://www.gschoppe.com/projects/fbformat/
http://slothsoft.net/UnicodeMapper/
http://qaz.wtf/u/convert.cgi
UPDATE July 2019 Facebook now lets you add limited formatting to "Group" posts.
![Screenshot shows formatting options for Group posts](https://i.stack.imgur.com/J7jIE.png)
If you click the back to front P type character in the update box, as shown in the screenshot, a few formatting options are displayed, "H1", "H2", Unordered List, Numbered List, Quotation. Also if you select any of the text you have written in the group post box, two more formatting options will pop up above the text, "Bold" and "Italic". These options are not available for comments on group posts, just the posts themselves.
Best Answer
As you are using Firefox you can install the Better Facebook! Firefox Add-on.
Firefox Add-on Link
The add-on provides numerous features/enhancements. One of these being
This should provide you with the functionality that your require.
You can actually install this on Firefox, Chrome, Safari & Opera. The website for further information can be found here