How are you accessing the site? Are you using a bookmark you saved before, or typing the address, or doing a search and then following a link?

Before diving in too far, I suggest running several malware cleaners in case your system is infected. You can find a number of free tools listed in our support article: Troubleshoot Firefox issues caused by malware.

I am accessing the my banking website by the address already typed in the address bar when this similar address appears and won't let me log in to my site. This fake address is also in the drop down menu on my tool bar, but I don't know how it got there

Does it look like a typo?

As you type in the address bar, Firefox should display various matches from your history and bookmarks. The bookmarks have a star icon (if they're open, they may have a little tab icon instead). Do you think the unwanted site is bookmarked?

To remove a suggestion from the address bar drop-down, use the keyboard: press the down arrow until that item is highlighted, then press the Delete key. Firefox will record that you never want to see it suggested again.

Just to be safe; You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.