The instructions are in the Clearing cookies, private data, history, and settings in Firefox for Android article.

Hi rjones244:

Waka_Flocka_Flame was correct (thanks @W_F_F!) to point out that article but it is slightly out of date. The actual instructions are:

1. Tap 'Menu' (on some devices this is along the bottom edge of the device, on others it is a software menu on the top right)
2. Tap 'Settings' (on some devices you may have to tap 'More' first)
3. Tap 'Privacy'
4. Tap 'Clear private data'
5. To delete history, check 'Browsing & download history'. Uncheck anything you don't want deleted.
6. Tap 'Clear data' to clear what you checked in the previous step.