Cookies: A cookie is a bit of information stored on your computer by a website you’ve visited. Usually this is something like your site preferences or login status. Cookies are all stored in the cookies.sqlite file.

If Firefox never remembers history, nothing is saved in history.

Firefox uses two independent cookie jars, one for normal mode and one for Private Browsing mode that is shared among all PB mode tabs and windows. During a PB mode session or in a PB mode window cookies are kept in memory in a separate cookie jar just like other session data is kept in memory. Cookies saved in normal mode can't be accessed as they are stored in a different cookie jar. The PB mode cookie jar is cleared when all PB mode windows and tabs are closed.

so can a username on gmail username be viewed if you go into the cache file?

In PB mode never as the disk cache isn't used. You can verify that on the about:cache page. In normal mode web pages you visit will be stored in the disk cache, so any Gmail page in the cache that shows the user name will reveal this data if you would recall this page, possibly in offline mode.

so if i have my browser on never remember history, will it remember anything?

Never remember history means that you use Private Browsing mode and in PB mode no personal data from the PB mode session is stored on the hard drive. Only in normal mode (Remember history and no checkmark on Always use Private Browsing mode) you could possibly find such data in the disk cache.