Website Tools
["View" > "Page Source"] A primary learning and troubleshooting tool (most any browser has this). Let's you view the raw HTML of any webpage in your browser.
["File" > "Save Page As" > "Web Page Complete"] Let's you see the other files that make up the webpage. Images and often "CSS" files are usually an integral part of webpages. You'll usually find them in a folder that is associated with the *.html file.
Firefox Extensions
FireFTP: A small Firefox extension that has all the FTP features you need.
LinkChecker: Checks the links on a page to see if the address is correct, and that they still exist.
View Rendered Source Chart: Reveals the structure elements of the webpage
Editors: see Editing for HTML Editors
FTP Clients - upload/download files:
FireFTP -- see under Firefox above: This is a full-fledged client.
SmartFTP -- a modern FTP client, free and paid options.
Link Checking:
LinkChecker -- see under Firefox above:
Special Tools:
SHTML Viewer -- if you want to use SSI (server side includes) -- which I highly recommend -- to build your website you'll want this essential tool. It combines your *.ssi files with your *.shtml file and presents it to your preview browser just like a Web server would.