Why can't i change my default language
I tried changing the language that it came with, which is Spanish, to English
In order to change the language of Firefox user interface, you have to uninstall current version (Spanish), then download and install English version.
All languages are available from this page: Download Firefox full installer for all languages/systems
Well said. Yu can also try;
Type about:preferences#content<enter> in the address bar.
On the bottom of the page, find Languages. To its right, press the Choose button. Make sure the correct language and district is selected.