I recently discovered a Firefox plugin that I already consider indispensable. FireFTP acts just like a regular FTP client, except it works entirely within your browser.
It has all of the features you'd expect from a standard FTP programme, including an intuitive interface; viewing of files by name, last modified, etc.; and CHMOD (i.e. change permissions of files on the remote server).
"So it's just an FTP client that runs within a browser; what's so great about that?" I hear you ask.
Well, it spares you those agonising seconds -- sometimes minutes -- that it takes to open a separate FTP client if, like me, you always have a browser window open.
Now if only someone would release a Firefox plug-in HTML editor as good as Homesite or HTML-Kit, and a CSS editor as good as TopStyle...