Have you looked at this extension?
About this Add-on
This add-on enables desktop mode by default; that is, "Request Desktop Site" is always checked whenever opening a new tab. This is designed for users with larger screens (primarily tablet users).
NOTE: Make sure "Request Desktop Site" and/or Phony are disabled when viewing this page; otherwise, you may not be able to see the install button.
EDIT: Take this tutorial you can:
- In the Firefox address bar, type "about:config" and press Enter.
- Tap the "Add a New Setting" button, then select "String" as the type.
- For name, enter "general.useragent.override."
For value, enter the browser user agent string you want to use. You can do a Google search for the user agent strings of your preferred browser and OS. In this example, we're using the user agent string for the desktop version of Firefox 17 on Windows 7.
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:17.0) Gecko/20100101 Firefox/17.0
Restart Firefox and confirm that Web sites are loading the desktop versions. That's it. If you want to change the user agent later, go back to about:config and search for "general.useragent.override." Hit the Modify button to change the user agent or the Reset button to remove it.