Open a Write Window From the Menu Bar select View-Toolbars-Formatting Bar

No Menu Bar in the Write window? Press the alt key.

I believe you have to be composing emails in HTML for the Formatting Bar to appear and work. It does not work when doing Plain Text.

Is the toolbar enabled?

Open a new message and in the Compose Window,

View|Toolbars|Formatting Bar needs to be checked.

Is HTML enabled on this account? If you're composing in plaintext then you won't be offered HTML-based formatting tools

Tools|Account Settings|<select account>|Composition & Addressing|Compose messages in HTML format needs to be checked.

Note that if you're replying to a message that was sent to you as plain text, Thunderbird may offer to compose in plain text when you reply. Holding down shift whilst clicking on Reply will toggle to the non-default state. There are also tools and add-ons that offer the ability to always use HTML, regardless of the original message's type; would this be of interest to you?